Almost got it right but no cigar. There is more to it than that, namely the faster miners are using better grade chips that can run faster and still be stable. Same thing is done by CPU makers, the various speed CPU's in any 1 family are not different - they are the exact same chip design that during binning tests are able to run faster than others. No matter what they are all chips have design targets that state min, avg, and max performance to be considered a usable chip and the chips are graded and priced accordingly.

Sure software tweaks can make a miner run faster than what the mfrs sold it as, question is, how stable is it? Same applies to OC'ing your CPU or GPU's.