I've seen this too. It's not just the activity feed. It also translates to things like the top contributor lists on the individual apps (Galleries, Blogs, Forums, Wiki). So I think the answer is that the system isn't currently instant at all times, but hopefully
we'll get it closer toward perfection in the future. (It's a rather new implementation, and we're trying to get different software tools to be compatible, which takes some time.)
Thanks!
Ed Price a.k.a User Ed, Microsoft Experience Program Manager (
Blog,
Twitter,
Wiki)