I agree. You said you added more water to help for boil off... Did that extra water touch the grain as well? Aka sparging? Or did you just top off with water before the boil, then also top off with more water in the fermenter? If you didn't "sparge"... Meaning you just added water to the wort before boil, then that could be the problem.
If not, all-grain brews typically are full-boil. That means you don't add water into the fermenter to reach a certain volume. Extracts are often partial boil, and then you top off in the fermenter. With grains, I don't know many people who do partial boils. You'd need more grains, as mentioned, to get a MUCH higher gravity, then add water later to even it all out