Achievement counts are cached for 15 minutes. It's really database intensive to fetch that information on every user profile / user hover / forum post.

And also, keep in mind that your achievement count will not be the same as the old system.

and are you sure your count hasn't increased? It grabs all of your current avatars, so if they are green on the page they count toward your total.

There was a period where some parts of the site were displaying the old number and then were moved to the new number (which you probably had 5-10 less achievements in).