Low Post Boil OG

Odd thing happened last Friday when I brewed an Rye PA. I had a pre-boil gravity of 1.050 on 7.5 gallons of wort and a post boil gravity of 1.056 on 6.5 gallons of wort. I had a pretty solid rolling boil and I am perplexed as to why I had such a low OG. I was shooting for 1.062-ish on this one.
Grain bill:
8.5# 2 Row
2.5# Rye Malt
1.25# Crystal 60

I did a standard batch sparge, but had about a 1/2 to 3/4 of a gallon of wort remaining in my mash tun as I am still in the process of re-calibrating my system from moving from a false bottom to a braided tube. In any case, I made sure only 7.5 gallons made it into my boil kettle prior to my boil. With roughly six gallons going into my fermenter.

Any idea how my gravity could have been 6 points off (and to be fair that 1,056 was a very liberal estimate). My hydrometer has been calibrated with distilled water as well.

Your answer is right there - it’s the extra wort left in the mash tun. Based on boiling off 7.5 gallons of 1.050 wort, you should have ended up right where you were - 6.5 gallons of 1.057 wort. Had you added the extra volume to get to 8 gallons of 1.050 wort and boiled down to the same volume of 6.5 gallons, you’d be right at 1.062.

Thanks Porkchop that makes perfect sense!