AI performs poorly despite decent depth

[AngularMomentum]

I implemented a chess engine and had it play a game against itself. It searched a maximum of 500 million nodes(takes about a minute) and searched to a depth of 8-10 in the middle game(7 was the lowest) not counting the quiescence search. I didn’t use any pruning that had a chance to change the result. My evaluation function is a piece square table with a 10cp side to move bonus. Despite the decent depth, the AI only had an accuracy of 75% according to chess.com. Is this normal, or is it likely that my program has a bug?