Just a quick dash of logic here. Weren't people modded for their dedication to something (e.g. Spending lots of time on certain forums) which means that a mod would most likely to spend more time logged on then a normal member. Also some mods have a bit of work to do so are most likely to be on longer so they can get it done. People don't go to schools at 4pm and wonder why there are so many teachers while there is a lack of students.