OG much lower than expected, but post mash gravity was close

[Hops and Brains]

This weekend I brewed an IPA with an expected OG of 1.061. My post mash gravity was 1.053 (expected 1.054). Pre-boil and post-boil volumes (6.5G and 5.5G) were as expected. How did I end up with an OG of 1.056. I'm no scientist and relatively new at brewing but the boil evaporates the water... not sugar. So shouldn't I have been closer to the expected OG if my wort volume reduced as expected? I don't recall ever being this much off before if my mash gravity was right.

Additional notes: I'm using Beersmith for making recipe and calcs. Equipment is 35L Brewzilla.
All grain. Just 2 row and maris otter. No sugar additions.
This was the first time I milled grains at home and think I went a little to fine with a portion of the grist which led to some pump clogging. But got through the mash alright and was very close to the expected gravity at that point.
I used only a refractometer for checking my mash gravity. Checked it while hot (1.052) and retested when the sample had cooled, which was the 1.053 reading. Used the refractometer, a hydrometer, and tilt for OG (1.056). All 3 were the same for OG.

 

[Hops and Brains]

Found a similar thread after posting this. Help with inexplicably low original gravity

Some good responses I need to look into. #1 would be stratification. I sampled post mash, after sparging, from the bottom, where the spigot on the Brewzilla is, without mixing the wort first.

 

[hotbeer]

If your pre-boil volumes and SG were as expected. And your post boil volume was as expected, then I'd think there was a measurement error somewhere.

Or perhaps you changed a setting in the software for something that gave a bad calculation.

Maybe even mixed up Imperial gallons and US gallons if your software allows that distinction.

 

[VikeMan]

53 x (6.5 / 5.5) = ~63

So, your post boil gravity should have been about 1.063.

If you measured 1.056 post boil, here are the possibilities:

- your pre-boil volume was not really 6.5 gallons (i.e. it was smaller)
- your pre-boil gravity was not really 1.053 (i.e. it was lower)
- your post-boil volume was not really 5.5 gallons (i.e. it was larger)
- your post-boil gravity was not really 1.056 (i.e. it was higher)

i.e., there's a measurement error somewhere. This is not a software issue, unless you simply trusted the software for one or both volumes, without measuring. (And even then, it would still really be a user issue, i.e. parameters need to be set correctly.)