This is because of the way Lemmy solves the "missing responses" problem. ActivityPub doesn't mandate a specific way to federate likes and responses, but the way Mastodon does this causes every server to have different replies and likes.
Lemmy forces the comments and likes to federate by boosting every one of them from the community actor, therefore ensuring that every server sees every comment. On Mastodon, you only see comments made by people someone on your server follows.
This leads to another rather annoying Mastodon problem, where someone will ask a question or pose a problem and will get a million of the same replies because everyone replying thinks they're the first to come up with an answer (as their server shows a small subset of replies). It can look like a storm of reply-guys and probably doesn't help Mastodon gain any popularity.
Lemmy and Mastodon both do this correctly from a spec point of view, but it's an example of how two correct implementations of the same spec can still have trouble interoperating.