Maybe because there are more distractions on a computer screen, whereas in real life, you've got a lifesize board there and a human being sitting across from you!
Try expanding to fullscreen. I've done this in the past. Sometimes I get the urge to click around- even if it's a great game going on. It's probably a psycological thing.
If this isn't part of your problem, I was still happy to help if it was :-) Good luck!!
I have a problem. I just joined this site and have a membership. If I play a live gamne I can play 1700 - 1800 rated players, but on computer chess I make these silly mistakes, blunders, ect. Has anyone else experienced this? It's really discouraging for me!