Uncommonly Low Gravity?

Alright, so today's brew day has ended with an unexpected twist. If you have any ideas as to what might cause an unusually low OG I am all ears. Here's how it went down. I've attempted to do a twist on a personal favorite. I've brewed it before with excellent results. It's Vinnie Cilurzano's recipe for Pliny the Elder. I mash using 1.25 quarts water per pound of grain. So for 14.45 lbs. of grain I added 4.5 gallons of water at 158*F. The recipe is below. Today I brewed this again.... and my measured OG ended up being 1.020 at 86*F. That ends up at 1.023 SGU. The mash didn't stick, I got an ordinary amount of wort out of the lauter tun. My mash temp was 152*F and I mashed for 80 min. (Boiled a prickly pear mead in the meantime). After the boil I ended up with an uncommonly low amount of wort, but I'm pretty sure that should increase the density of the wort; therefore raising the specific gravity. I'm at my wit's end and am feeling a little guilty as today's brew day was not beer for me, but one that my friend paid for so that I could walk him through the process and show him how it's done. The recipe is below. I believe I've given all the useful facts as well as some useless information. And if you've made it this far in the post, thanks already!


Target Stats: Measured Stats:
OG: 1.072 Actual OG: 1.067
Extract efficiency: 75% Real Efficiency: 84%
IBUs: 90-95 Actual IBUs: 265
ABV: 8.2% Actual %ABV: 7.5%
SRM: 7

Malts:
13.25 lb – 2-row pale malt
0.6 lb. – Crystal 45 malt
0.6 lb. – Carapils (Dextrin) malt
0.75 lb. – Dextrose (corn) sugar

Hops:
90 min - 3.50 oz. Columbus 13.9% AA
45 min – 0.75 oz. Columbus 13.9% AA
30 min – 1 oz. Simcoe 12.3% AA
0 min – 1 oz. Centennial 8% AA
0 min – 2.5 oz. Simcoe 12.3% AA

Dry hops:
12 - 14 days = 1 oz. – Columbus
1 oz. – Centennial
1 oz. – Simcoe
5 days to go in dry hop = 0.25 oz. Columbus
0.25 oz. Centennial
0.25 oz. Simcoe

Yeast: WLP001 or Wyeast 1056 (2nd attempt: WLP530 & Wyeast Brett. B.)

Directions:
Mash grains at 151-152*F for 60 min. Mash out at 170*F and sparge. Collect 8 gallons of runoff, stir in dextrose, and bring to a boil. Add hops as indicated in the recipe. After a 90 minute boil, chill wort to 67*F and transfer to fermenter. Pitch 2 packages of yeast or a yeast starter and aerate well. Ferment at 67*F until fermentation activity subsides, then rack to secondary. Add first set of dry hops on top of the racked beer and age 7-9 days, then add the second set. Age five more days then bottle or keg the beer.

Today's Notes / sources of error:
1) I sparged with 2 gallons room-temperature water. I've done this before with good results, so I stopped worrying about sparge temp.

2) I'm brewing in a new boiling kettle with a wider mouth. I understand that this explains the higher evaporation rate, but I do not expect that it would lower the gravity.

3) I mashed for 80 minutes instead of 60 min. This couldn't hurt... could it?

4) My thermometer is a cheap digital one and it's got a whimsy that is entirely its own. I've checked it though, and when it gives a steady reading it is properly calibrated.

5) After the boil I ended up adding sterilized water to bring the volume up to a solid 5 gallons. This certainly would lower the gravity, but I did the same thing last time I brewed this one, and I hit 1.067 SGU.


Any and all ideas will be most appreciated! Thanks!
James

Please feel free to check/correct my math. I think it's all right, but what do you think?

Is your thermometer accurate?

You process looks ok.

>>5) After the boil I ended up adding sterilized water to bring the volume up to a solid 5 gallons. This certainly would lower the gravity, but I did the same thing last time I brewed this one, and I hit 1.067 SGU.


Did you did mix in this water, before you took the gravity reading?

I guarantee you, adding water and not mixing it in, will distort gravity readings. The water does not automatically mix itself.
I have experienced this first hand.

The only other possibilities are - your thermometer was way way off, or the grain was poorly milled.

The reading was steady on the thermometer when I got the 86*F reading and gravity. Also, if my math is correct then the wort would have to be 207*F to give a hydrometer reading of 1.020 with an actual specific gravity of 1.061. That's not a discrepancy that's easy to overlook with your hands on the kettle. I can't be certain (Like I said, I definitely need a new thermometer), but I think my thermometer gave me an accurate reading.

@Arclight: I think you may be on to something. I did not mix in the water before my gravity reading. Having not done this before, it didn't occur to me. At the very least this is a gleaming light of hope. BTW, this evening the fermenter is bubbling away quite healthily, and boy does it smell good! I will have to post back and let you know how it turns out.

I think Arclight's probably right. Any other thoughts would be most appreciated!

I'm with Arclight on this one too, we see the same complaint all the time in the extract brewing threads.