Perhaps it's just that, due to the extra time, your opponents are punishing your blunders more often in online chess. If you play online and live nearly the same way / speed, then this would seem to be the answer. In this case you blunder equally as much, but in your live games you don't notice because your opponent also missed it.
Blundering more in online than in live chess.
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For me it seems that I blunder my pawn or queen in online chess than on live chess. Do you feel that way also? Because when its my turn I see my opponent has taken my queen and I didnt even know he could have. I dont do that in live chess. So its very weird.