I heard that universities look at marks differently depending on the high school, is that fine?

Yes, some schools apply mathematics to the academic average you provide, based on internal metrics and expectations. Graduates of some high schools may have their average adjusted up, while others may have it adjusted down. The idea is to level the playing field as much as possible so universities can directly compare students, without worrying about grade inflation or deflation. There’s nothing you can do about this, really!