the computer assumes your opponent is the computer and will therefore play the superhuman moves that it sees. Your opponent is presumably a human, who didn't see those lines and managed to lose.
This is pretty normal, really, assuming you are not a GM.
Computers are following their parameters, so sometimes, they mess up and evaluate the position incorrectly. However, you should not underestimate their strength.
Or you played a bad move, but your opponent did not see the computer's move so he is lost.