I have members on my list from the old Funadvice format and have keep them on my list as a sign of respect and if they do (fingers crossed) log into Funadvice again, I will know they have been here. People I don't interact with, I remove them after 1 month if they are not here.
Maybe they don't notice that the person hasn't been on in over a year. Or, maybe they saw great advice made by that person on an older post and decided to follow him or her, despite the fact that he or she isn't very active on the site anymore. Who knows!
I don't usually keep track of ALL the members I follow. If they haven't been back in a while though, it may be because they've gotten busier or forgot about this place.
Actually, people from a year ... even two years ago still come back from time to time.
They probably didn't look at the last time they were on or just didn't care.
The person probably just never bothered to remove or unfollow the person.
thats good to know :)