Hi @Pikapika_C, welcome to the forums.
Zwift racing is full of people in the wrong cat, we call them sandbaggers. It is easy to assume that they are just cheating and they enter a lower cat for an easy win, but it could just be that they have no idea how the different categories work, or a language barrier. You grasped it quickly, so why are they not able to figure it out too right? This is where zwiftpower.com comes into the picture, a 3rd party site that “polices” the races by ranking only the zwifters who are in the correct category. You should sign up and link your Zwift account if you plan to keep racing. On zwiftpower those who are over the cat limit as you described above will get filtered out of the results and DQ’d.
Zwift is currently testing a system to help get users into the proper cat as this issue has gotten out of hand, and has been a huge complaint in the community for years it seems. Check out the articles on ZwiftInsider.com to learn more about the changes being tested.