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By advantage I mean posts from those instances receiving more visibility than others on feeds that sort by score (active, hot, top).

There seems to be at least two ways in which posts from instances that don't allow downvotes receive an advantage:

• They don't federate downvotes. That means other instances only count downvotes from their own users but not from the rest of the fediverse.
• A downvote sometimes can be counted and federated as an upvote. This happens when you first upvote a post and then change it to a downvote.

Let's see an example. Suppose we are a user from instance A that allows downvotes and we want to vote a post on instance B that doesn't allow downvotes. Watch what happens on instance C that also allows downvotes.

1. Before the vote this is what users from each instance see (upvote - downvote = total score)
   A: 10 - 0 = 10
   B: 10 - 0 = 10
   C: 10 - 0 = 10

2. Now we upvote the post:
   A: 11 - 0 = 11
   B: 11 - 0 = 11
   C: 11 - 0 = 11

3. We misclicked, we meant to downvote the post:
   A: 10 - 1 = 9
   B: 11 - 0 = 11
   C: 11 - 0 = 11

If the post was hosted on an instance that allowed downvotes users from instance C would see a total score of 9.