Game Center game data vs iCloud game data

I have saved game data (saved progress) on device A with iCloud user A and Game Center User A. On Device B with iCloud User B and Game Center User A, I see all user A’s ‘achievements’ but no save / progress data in my game. This is extremely frustrating. What is the use case for allowing a Game Center ID to be on another device under a different Apple ID user account if all they see are achievements?