I noticed that too, I imagine it’s something to do with the Lemmy software, I have no idea how it is calculated on a technical aspect, but I generally just ignore it as it’s mostly unhelpful.
https://github.com/LemmyNet/lemmy-docs/blob/main/src/contributors/07-ranking-algo.md has some info on that but seems to be focused on post/comment sorting. Maybe community sorting uses a similar mechanism but this isn’t clearly mentioned in the documentation.
edit: trending communities are local communities sorted by “hot” ranking, which is calculated using the logarithm of the number of subscribers and pulled down by the time the community existed.