You are talking to other people, unlike in RL he can watch the way you are interacting and knows what you are doing. On the computer he has no idea unless he is reading over your shoulder or you are sharing the stories with him. My bf, always asks what I am doing when I'm on the computer..I always say I'm on my forum. I have never explained what it is or showed him and he has seen the page up but hasn't investigated any further but I know he's just that little bit curious...he's not very good with computers and I haven't enlightened him because I don't think he will be interested anyway.
It's more that you have something that he isn't sharing...your mind is occupied as it needs to be when answering some of these threads. I've felt the same...hang on a minute I have to finish my train of thought before I go about anything else or it gets lost!