Hello.
Remember that the important thing are the GLOBAL statistics resume.
I mean: if your forum has more users than other forum, that doesn´t necessary means that your forum must be first and the other forum second...
The important thing are GLOBAL statistics...I mean, for example, if you have temporary closed your forum (for redesign reason or whatever) and later you opened it aggain to activity, then it makes your statistics go down, even you have more users/posts/topics than others forums.
Imagine also that, for example if you have 100 registred users in one month, but other month you only have 10, then it makes GLOBAL statistics go down.
So, if the other forum, despite of having less total statistics has not stopped in any moment it can overtake your forum.