I don't think it was purely a calculation error, it was probably also an intuitive evaluation. Bobby was trying to complicate the game and create an imbalance, where there were still winning chances if misplayed by spassky, but obviously it was a bad evaluation and it backfired. I think he was in a bit of a mood and got reckless. He had been making a lot of demands leading up to this and threats to not participate, probably got frustrated by the drawn endgame and took a big risk. I don't think he ever really opened up about his reasoning to be fair, but was asked along the lines of "were you trying to complicate the game" and he said "something like that". After losing those first two games he demanded the cameras to be removed from the playing hall and started to play really well against spassky, so possibly a psychological aspect from the cameras were also to blame. Maybe he knew he was throwing the game and it'd make for an entertaining match... the guy was sort of insane