Edit: obligatory explanation (thanks mods for squaring me away)…
What you see via the UI isn’t “all that exists”. Unlike Reddit, where everything is a black box, there are a lot more eyeballs who can see “under the hood”. Any instance admin, proper or rogue, gets a ton of information that users won’t normally see. The attached example demonstrates that while users will only see upvote/downvote tallies, admins can see who actually performed those actions.
Edit: To clarify, not just YOUR instance admin gets this info. This is ANY instance admin across the Fediverse.
In a decentralized but federated environment, sharing data is inevitable. In this case it’s important to only share what’s needed when communicating with other instances. For example, if you visit a community through your own instance while being logged in, does the other instance knows about your account being logged in ? It’s not needed, but this information could leak, and this affects your privacy directly. Because what you post is public, but what you visit should be private (or at least, limited to your instance).