Often significant variations exist between drivers within any given batch. Manufacturers have tolerences of difference from the ideal in order to operate within fiscal limitations; that is, they can't throw everything not precisely to spec away, because they'd be throwing away the vast majority of their products. As such, they employ driver matching, a process which seeks to align the sound reproduction capabilities of a given driver in the batch with another (whether from that batch or not).
For lower end headphones, they usually only match at one or two points (often 1000hz, to align the upper midrange, both areas which the human ear can detect); for medium level cans, this is done at a few different points (usually 1000 and multiples thereof), and for the flagships and other expensive cans you can expect relatively close full range matching. Grado promises 1dB to 0.05dB deviation, Sennheiser promises 3dB on the HD580 and 1dB on the HD600 and HD650.
In short, manufacturers' tolerances adequately explain difference between headphones of the same brand and model.