All it shows at the moment are the first posts for topics you made. So it's exactly the same thing for "messages" and "topics."
They're the exact same thing, that is, except for the fact that "show posts" still lists the number of pages you'd need to view all of your posts. It's very noticeable with my profile, since I've only started about five topics. It shows the five posts I had as the first page in a topic on the first page (numbered 1-5), and those same five posts on the last page (numbered 20000+, based on what I assume is my current total post count), and in between I have over 1300 blank pages.
It's definitely broken in some way- almost reminds me of how unregistered guests to the forum are only able to view the first post in each topic, so it will only show posts that were the first post in a topic.
Anyway, that'd be a very nice feature to get working correctly, if you get the chance to tackle it >.<