I'm assuming you're talking about BL specifically. Personal theory ahead. I think manga readers are less likely to give 5 stars in general. If you're a manga reader, chances are you've been reading BL for many years now (there are exceptions), you've already seen it all and barely anything surprises you anymore so you won't give everything you read a higher score, even if it was pretty good, but manhwa has only become a thing in the last few years and a lot of new BL readers start with it, to them everything is new and entertaining, so they'll give anything with a nice art style and character chemistry 5/5.
To be honest, rating systems kinda went downhill everywhere. People used to rarely give a full score back in the day to movies/anime/books/manga. They were keeping it for something special, an ultimate favorite they considered "flawless", but nowadays people give everything that kept them entertained and away from social media 5/5 or 10/10. I can't trust any "Top" on any website anymore.
how come mangas get such low ratings compared to manhwas? the peakest peak manga will get like 65% or 8.5 stars nb then slop manhwas get 72 easy