The forum list does not change when you have more users in a day.  It's cumulative with total activity in a forum over a period of time, which doesn't change often.

The forum ranking system is based on a number of factors, some obvious, some not so obvious. We deliberately do not give out details, because some forums would then try to "game" the system to get a higher ranking, and that often makes it a much less fun forum to be part of. Our advice is to make your forum as interesting and rewarding as you can and  ignore the rankings.