The difference is that at work I am forced to look at that screen, even though I would rather go for a walk, do sports, etc. when the sun is still shining. I must give my employer the illusion that I am working, even though it would take me two hours max to do the work they give me. This has been the case in every job I ever had.
At home I stare at the screen to keep my girlfriend under the impression that I am working, because otherwise the is getting on my nerves constantly. I can read books only in secret.
Sometimes I take a walk between work and arriving home and say that I needed to do more hours.