Shelagh, your profile says 6356 and the number under your name says 5889. I know you have been on here a while but I doubt that you have sent over 450 PMs in that time. If I recall correctly, posts in some sections of the forum, like this section, do not change your count. I am probably wrong though. It may be that the counter on your profile is counting everything.
Edit: Wrong again I guess. I just checked my own numbers before and after this post and found both had gone up by one. I also looked at the difference between my two totals and they are much closer to each other than the number of PMs in my present in-box(you do end up with quite a few PMs as a MOD). My numbers are only apart by 6.