The Admin Userlist Addon is not doing the job perfectly well. I´ll try to explain:
Recently joined users that have sent PMs won´t show up on the top of the list when sorting for PMs. They will still show up at the end of the list. It seems like their joining date overrules their PM count. Nevertheless, when sorting for PM count, older users with sent PMs will show up on top of the list when sorting for PM count.
On my board there´s little PM traffic and so it happens that when sorting for PMs me as the Admin shows up on top and a recently joined user with PMs sent shows up almost at the end of the list.
Was this clear enough to make you understand the bug in the Admin Userlist Addon I seem to notice?
If not, please ask me to be more explicit.