It's all about what the anti-cheat engines look for, they're like anti-virus engines or heuristic scanners. They can scan for certain bytes (scan strings) and if they find one it means a hack has been detected.
Or they can analyze the behavior of modules, if one is trying to set your health to 1.0000 where you can have 100 health as a maximum...
That's what they do, you either have to disable the anti-cheat engine, or finding an undetected way in (bypassing)
-SCHiM