Or they could be using totally differnet frame rates and timing arrangements.

it's actually more surprising that all the various brands of transmitters using FM-PPM actually do work with each other's gear (except for shift direction). I think that's going to turn out to be the exception rather than the rule in the long term. Between PCM and 2.4ghz, I suspect the days of mix-and-match TXs and RXs may be numbered.