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.
Except it is not just your instance and the instance hosting the post (which would make sense). It is every instance in the federation. So if I want to see how you vote, I just need to pay few bucks for hosting and make my own instance.
Actually, it’s even easier than that, just make a kbin account then go to any Lemmy instance and you can check the upvotes/downvotes.