It's call variance and it's normal
Except when it isn't. There are many factors that go into the ultimate performance of AntPool. Luck
is just one of them. It is also possible that they have problems from time to time.
Such as?
All kinds of things can cause problems. Power outages, Internet problems, software/hardware problems, fire, flood, or other disaster, hackers and attackers, and lazy incompetent operators. Basically everything has to be working perfectly for it to boil down to simply luck and statistics.