I constantly encounter some inconsistencies in counting posts due to forum ranking parameters, and now I am communicating with the admin. So I hope the problem will be fixed in the near future.
Dont use activity for counting posts, use number of posts instead, that information is not available in main profile pages.
You should go to the last page of showposts to see the total number of posts you need.
It would be super cool if someone could add this info to telegram bot.