It's fine if they all have different calculations to estimate rank. Apps will use whatever users want as "rank". Basically different signing keys for TAs can have different behaviors but they will all be used in the same way by the client.
What clients need to know is that rank is the score of the user, and "followers" is the follower count in whatever way that provider defines it.
It's kinda similar to DVMs. The NIP defines a contract between client and provider but after that the provider can do whatever it wants.