Your post numbers will fluctuate from time to time for various reasons. If the site updates, it's possible that some of the info to your profile does not transfer. Group Mods will periodically go through their groups and glean their group discussions and delete some topics that are old or not very active. You can lose post numbers here as well. If a group is deleted or disbanded or a group Mod closes down groups and then leaves the site, you'll lose those post numbers as well. If a forum topic is found to be negative, uncivil, too sexual or not to the site owners taste, that topic can also be deleted and you'll lose post numbers there.
I've said this many times; it's not the number of posts you post or the ranking you have under your profile that counts, it the content of your postings. There are several here with Ultra Nudist rankings and they contribute nothing to discussions but rude, uncivil comments. Worry less about the numbers of posts and rankings and concentrate more on contribution to topics of discussion with content and less about piling up numbers of posts. That's what's really important.