I have a profile pic that is different. But that is just how vBulletin is. You can have one that displays on your profile only and one that is for posts.
By default, I'm not sure it's a feature of vbulletin, but might be a setting possible through permissions... either that or I believe you can get a plugin that does it but, probably not worth the hassle in the big scheme of things that need to get done. Honestly couldn't tell you why they separated the two, I've been on various forums using it since 2006 and never figured it out.
If you want the "why is it like this?" answer, sometimes people use their real photos for a profile picture, when people visit their profile, but when posting prefer not to have their face totally plastered everywhere.
You can see how mine is. It's also like that for me on all the other vbulletin forums im a part of
I'm confused as to why it matters if it's a pointless feature, it's there, and it's pretty lowkey if you ask me. It's not something we're going to add to the In-Development in quite some time.