That happends to me too. I think it's that you're not playing your full strengh (i.e. not taking the time to find the best move, playing by instinct, playing subconciously, etc.).
The Computer
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I think it's definitely much stronger especially in the opening. It makes stupid mistakes later on in the game, but it plays strong in the beginning.
When I play against the computer here at chess.com, I can`t seem to win though it`s set at ~1200. At turn based chess my rating is ~1500. Am I just not that good or is the computer stronger than it`s rating