i think it's that different hubs have the same threading, only some are more touchy and finicky than others. hence it's best to pair manufacturers when pairing cogs and hubs. phil hub = phil cog, dura ace hub = dura ace cog, etc. i don't think the threading is too different where a phil cog totally cannot go on a dura ace or campy, but i think the threadings are just a touch different. (disclaimer, i have always only ridden on my couple of wheelsets and haven't much experience with other brands, so i might be way off).