you shouldn't act like that, infact you should be happy to get to see and hear what your members/visitors really think about your site and the way it is ran. Seeing this alone proves that you are either not active enough in your forums as much as the other admin or that you have not made a good post/reply to a thread. Other possibility could be that the other admin may write better and a bit more formal than you, and he may sound older than you therefore people might just assume that he/she is the owner.
My advice to you, be more active, make sure you start most of the major news and announcement posts, that way people will get a better idea of who is who.
At the same time, you should be happy that you have another admin in your forums who has worked probably just as hard as you to keep the site active and has spent time on the boards.
Again, don't be dissapointed, this can be improved over time