Sometimes they hate certain shows. I've mentioned several times about their review bombing campaigns, especially on anything that temporarily beats FMAB at #1. It's a bit sus when a show reaches #1 and less than an hour later FMAB gets back to number 1, and the other show mysteriously has a ton of 1 star ratings.
MAL doesn't really care much for slice of life shows, and you'll see with the shows that have multiple seasons, that season 1 gets like 6's or whatever, but season 2 (despite not doing anything different) will magically have a higher rating because only the people who enjoyed season 1 bothered to watch season 2.