So I an average player at on line (not blitz) /correspondence/turn-based chess.

I'm 1657 rated here. 1800 at ICCF and 1715 at chessworld.net, however my National ELO in OTB games is 1598 (I'm ashamed of myself). Now the question is why?!?

Is the ELO on internet chess sites too inflated? (I know that in zone.com it is) or I just have a different view of the board when I looking at a screen rather than when I look and actual chess board?

your otb rating will always be lower than online.

main reason is online you can use opening/game   books/dbases for research.

you have days rather than minutes to decide your moves 

also you can choose your opponents whereas in otb you are paired, usually with someone your own level. 

I also seem to be better in online chess.  I think it's because there's less pressure on you when you have three days to move as opposed to three seconds in a clocked game.  :)