PP - my take on it is they haven't had the numbers to the track just yet, I'll give him 12 months before judging.
That said they are totally different styles of trainers, Snowden is much more old school and goes by 'eye' (The bloke must be a genius) while O'Shea is more 'modern'. He also works them a lot harder.
Maybe the horses are adjusting to the different styles too?
With the jocks it seems like they are splitting the rides between McEvoy & McDonald and they are sticking with horses through out the preps, not sure who decides that though