Anyone remember the article from a year or two ago, when it was discovered that Apple was sourcing CPUs for its phones from two different manufacturers, and the phones containing CPUs from one source performed marginally better than the phones contain CPUs from the other source?
There was a big to-do, with people trying to figure out which iPhones were "the good ones", and people who had received (or thought they had received) the slower version were complaining and debating whether they ought to return their "inferior" iPhone in order to get one of the "better" ones.
Of course it turned out the difference wasn't really noticeable unless you were specifically benchmarking for it, but the fact that it was detectable at all produced a big (well, medium-sized) scandal and a headache for Apple.
Given that, I'm not at all surprised that Apple now aims for uniform performance across all units of a given model, rather than for best-possible-performance on any given OEM chipset. Uniformity makes everyone happy, whereas an optimal performance will go unnoticed by the people who have it and the people who don't will be pissed off.