Surely that’s just part of the code on each instance? If there were a bias it would have to be present there.
I don’t use any kind of platform besides this site but unless there’s some kind of global analytics system run by somebody affiliated with Mastodon that every instance is set to reference by default I don’t know how this would be possible.
I’ve always considered the federated stuff too clunky. Only data should be federated, presentation should be unified. As with newsgroups.