There's an avatar, and there's a profile picture. They don't have to be the same, and some people have set one and not the other. You've changed your profile picture, not your avatar.
I would have thought that there would only be one place to stick your avatar, but apparently, for some reason, the vBulletin people wanted the option to have to images. Thus, if I looked at your profile, I might see, say, your real picture. But your avatar next to each post would be your caricature.