Some play from mobile devices, others may log in, make a move and log off.
They aren't online long enough for the server to register them as being online.
Online and offline
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Fair enough! Thanks!
Another possibility is that conditional moves can be made. (from computer login) If it's obvious what your opponent will do (like get out of check) one may go ahead and make a conditional move and the computer will remember your move and make it for you if the opponent actually makes the move you predicted. I use my mobile chess.com mostly, but haven't figured out how to do conditional moves via the android. I have wondered the same thing before, and I think it's a good question.
I am fairly new to this site and I can't work out why some people make moves without ever appearing to be online! Is there some special reason or am I just crazy?