Feeds (RSS, ATOM etc) often times contain only a small subset of recent items, like 10-15 of them. This means that if you limit yourself only to the feed themselves, you will lose track of upvotes at some point, for example after a day or two past the publication time of a given item.

Obviously this is something you have thought through, so I assume you just treat item disappearing from the feed as some form of indication it's not "trending" enough from the source point of view. Am I right?