There also seems to be something strange with the calculation for your expected values.
The expected values say you planned to boil down 8.5 gallons of 1.063 wort to 6.5 gallons. This is a concentration factor of 8.5/6.5 = 1.031, so you'd expect to get (8.5/6.5)*63 = 82 points, i.e. an OG of 1.082. To reach 1.087, I'd calculate you'd need to boil it down to 6.15 gallons.
Did you get these numbers from Beersmith?
In the event, you had 9.6 gallons of 1.061 wort boil down to 7.5 gallons, which gives a concentration factor of the slightly lower 1.28. You should then get (9.6/7.5)*61 = 78, so 1.078. Like others have said, this is close to the 1.076 you report, and probably within reasonable measurement error on the volumes and so on.
It's worth noting that the volume is 4 % higher at boil temp than room temp, so if you're measuring one volume hot and one cold, you could need to adjust your math a bit. For instance, if your 9.6 gallons were at boil temp, and 7.5 at room temp, the difference is really between 9.6*0.96 = 9.22 and 7.5. Incidentally, using this, we get ((9.6*0.96)/7.5)*61 = 75, which is even closer to your 76.
Re-reading this answer, I'm not sure if I'm clearing things up, ur muddying the waters. At least I got to do some calculations!