You may have lost track of how many quarts you put in originally, happened to me once in a biab. I put a gallon or so more then I was supposed to and didn't catch it till the end of the boil. From that point on I always watched how much I had in the kettle as I was filling it, in your case with each sparge. You may have also double or triple sparged when you meant to single or double. Those are my best guesses, because if you sparged with 8.5 gallons you would have been left with around 7.5 gallons not counting dead space, so you should have been fairly close. If I batch sparge I have a gallon or so extra, mash 1.25 qt/lb, then find out how much I got out of the 1st runnings. With what I have in 1st runnings, I subtract that from the total I need. Then divide that by two because I double batch sparge. However never go off calculations, as you fill your kettle with the last sparge watch your amount and kill the last sparge if you get to much.