Because it is way easier to define a sentry being “a member who has at least Y(N) certifications”, both computationaly and humanly.
You don’t really bother of the Y function as a human, you just want to remember the values which are:
N Y(N)
10 2
100 4
1000 6
10000 8
100000 12
1000000 20
This is super easy to understand.