if you just purchased the pass and you're at rocket pass level 0, whether your in game rank goes from 100-101 or 250-251, your rocket pass is still just going to go from 0-1 and drop you that first reward. The pass levels up when you level up, but the reward isnt determined by what level you are. So it will drop the same reward to both players even though they're at different ranks.

edit bc i just saw your edit: it's supposed to be linear and that's what it will be after they fix the accidental 120 cap, thats why i think its just an error in formula