Help with grain absorption!

I have a no sparge system and I am finding my grain absorption is not the same with different amounts of grain. I am using a converted sanke keg with a sabco false bottom. This mash tun is bottom draining. I had a 1/2" coupler welded to the bottom. It also has a sight glass. I have double checked the markings on the sight glass to make sure they are correct. My last batch was a porter. I used 15.25 lbs of grain. This consisted of 2 row, crystal, black patent, and chocolate. I started with 8.5 gallons of water (this is no sparge). I was using the .12 gallons/lb absorption rate. This comes to 1.83 gallons. My actual absorption was about 1 gallon. The LHBS uses a crankenstein mill. The crush seemed to be normal and my original gravity was about what I would expect for the 7.5 gallons that was collected. What would explain an absorbtion rate of .065 gallons/lb.

Humidity?........

Is it possible that you accidentally used 9.5 gallons of water. That would have given you absorption of almost 2 gallons (15.25lb * .13gal/lb = 1.98 gallons) to get you to the final 7.5 gallons.

If you're 100% sure of the water, then I'm clueless.

I checked that twice. Not to say that's not possible. I wasn't drinking at the time.