While I haven't used it myself, I think gravitar is an avatar that gravitates toward you, as implemented by you putting an image on their server, and countless other websites include it from that server when an identifier (probably email address) match exists between the two accounts. So in other words, gravitar knows a bit about your usage of countless websites that each volunteered your usage pattern to it. Since evidently you are seeing a connection between your machine and gravatar, they get page-load granularity. If this description is incorrect, which it very well might be since I know nothing about the service beyond what I inferred by its name, please do correct me.