Consistently low OG

I am partial mash brewing using the 'Countertop Partial Mash Method'.

I am adjusting the ph of my water.
I am adding the recommended amount of water to my wort in the primary.
I am stirring the wort before sampling.
I am getting my wort to 70-75 deg before sampling.
Yes, I know how to read a hydrometer.
Yes, it is reading 1.000 (or very close to that) in tap water.
I brew in Austin, where the tap water is very good.

So I consistently seem to miss the OG by a little more than half a point. For example I just brewed a Milk Choc Stout. Target OG 1.071 -- I hit 1.064. However, I seem to hit the ABV numbers, so my FG will be a bit lower than expected as well.

Also, my beer tastes good, sometimes great.

I know - RDWHAHB. I am. (A nice IPA, that is astoundingly clear).

I'm more interested in the science of what is going on, and what could be causing my problem.

What temp are you mashing at and for how long?

Did you get a good crush on your grain?

When I did partial mashes and to a lesser extent extract batches, my OG numbers were always all over the place with no consistency. I was able to pin this down to a few common problems and improved my OG numbers a bit, but the only way I solved it was when I went to all grain, which may or may not be a viable option for you. Let me make a few assumptions here based on the "countertop" method you mentioned:
1) You are making a 5 gallon final volume batch
2) You are doing a partial boil of 2-3 gallons
3) You are adding DME or LME late and topping up with water to hit 5 gallons

Since you mentioned your hydrometer was fine and you are reading it correctly, my first thought is that the OG given by the recipe is off. You didn't mention if you are formulating the recipes, using others' recipes, kits etc. If you haven't invested in any brewing software like beersmith, promash, beertools, etc then you may want to look into it. Second thought, if you are using some software (or running the numbers by hand) to formulate a recipe, then perhaps your efficiency numbers are off. Many recipes are written with a specific extract efficiency in mind, generally 70%, and if your efficiency is less than that, then you need to compensate with extra grain, or more likely in your case, DME or LME to hit your OG. I would take a pre boil gravity reading and see how much sugar was extracted from your mini mash, then calculate out how much DME or LME you need to hit your target gravity at the end of the boil. You also need to keep in mind that as you boil, the specific gravity of the wort is rising as water is evaporating, and the specific gravity decreases when you dilute your concentrated wort with water in the fermenter. How I overcame this was by purchasing a refractometer and taking samples throughout the boil, and plugging those numbers into software or a spreadsheet than will take into account the dilution of the wort in the fermenter. In your example of the choc stout, if you boiled down to about 2.5 gallons and diluted with 2.5 gallons in the fermenter, to hit your OG of 1.071 (according to my math), you would have needed a post boil gravity of about 1.142 before you went into the fermenter. If you can, post your recipe you used for the stout and I'll take a look at it. I've got a feeling that your biggest problem is the dilution because that is the area that makes the most sense. You also didn't mention if you have good control or measurement of your final volume post boil and the volume of water being added to the fermenter. I ran the numbers again and with the exact scenario above, adding just 1/2 gallon more dilution water dropped the OG from 1.071 to 1.065, which looks very close to the numbers you are describing, so I would start there and get exact volumes and measurements under control first and go from there.

Joe

You've eliminated most of the potential problems, except for crush. If you are crushing at a LHBS and can't adjust the gap, run the grain through twice.

that method does two runnings (1st running and one sparge). i did that byo method a bunch of times.

i consistantly got 65% doing that. last weekend I did another sparge, and hit 72%. i think that might be where your missing gravity is. the last sparge i got 2 gallons of 1.026 wort. better that 2 gallons of 1.000 water.

Most hydrometers are calibrated to 59 degrees. The adjustment for that would give you a teeny bit more if you are measuring at 75 degrees. But maybe you did adjust.

I've been having difficulty with hitting my numbers exact, and realize part of it is water measurement errors. Things like the fact that water expands about 4% between room and sparge/boiling temps, the amount the hops absorb and leftover trub, the inaccurate markings on the 7 gallon bucket, and on my measure stick.

If you suspect you efficiency is the only problem, perhaps you should just adjust your amounts so the O.G. matches. Recipes can be based upon anywhere from 60% to 80% efficiency, so I always use a calculator to adjust for my efficiency.

Still, I'll usually be about 5% off, and I'm good with that. At 10% I"m looking, like you, but have recently found those pesky measurement errors. Like everyone says, consistency is more important than efficiency.

If your OG is low and your FG is low.. could it be your hydrometer isn't calibrated correctly?
Check the calibration using distilled water at the specfic temperature your hydrometer was calibrated at (60F)

This may be a longshot but check your thermomter's calibration.

I noticed 2-3 batches ago that my efficiency was going to ****. Turns out the thermometer I was using in my mash tun was reading almost 20deg high.

Weigh your raw materials. You might be surprised at the results. I have a digital lab scale that is very accurate and I find that sometimes DME and LME are less quantity than "I paid for". Once I got a whole pound less LME than I ordered. I have not noticed this error in the other direction....hmmmm...velly intelesting...

My thoughts are consistency is a good thing. Assuming you are using some type of brewing software, you could adjust your brewhouse efficiency down until your expectations matches your results.

I've got pretty consistent results with 65% efficiency myself. There's room for improvement in a few areas (crush etc.) but at least I end up brewing the beer I set out to brew.