It seems obvious once you look at it - the "efficiency" mode is almost a quasi-sleep and so it makes sense to only dedicate a certain number of processors to it (as every efficiency core could in theory have been a performance core).
How many is optimum is an interesting question, I would assume the apple peeps have a good idea and are on the money with this design.
At some point it's more efficient to have a single performance core going than a number of efficiency ones.
They do up to 8 on the big CPU according to rumor, so there's obviously some room there.