It's not that it's independent of performance. It's just that most govt agencies give you the default grade in the first year because they aren't able to fully assess performance.

Nonetheless you will get a performance bonus based on this default grade aka C. There are instances where officers are ranked in their first year but it really depends on their reporting officers' choice and the month which they joined. Let's say they joined in Jan - quite likely they would be explicitly ranked. However if they joined in Jun, then it's highly unlikely that they would be explicitly ranked.