Its simple, the ball bearing cartridge has a different mating surface/dimension onto the exhaust housing than a non-ball bearing cartridge.(slight larger diameter and offset)

When the turbo descriptions/comparisons are given they talk purely about the size/capacity of the housing.....leaving out the detail that there are slight differences in how they physically mate to each other with particular reference to ball/non-ball . However they are still the same capacity i.e P15 P18 etc etc thats not a part number, its a capacity description.

I have/have had several of the mentioned turbo`s, I have tried to bolt/slide them into each other....they only fit where compatible.

By all means ask what fits where if your not sure, and you will be provided an answer, Its very simple and not worth the craping on about so please accept the informed answer you receive