I seem to recall reading it was normal practice for the the
obverse die to be the lower one, on the anvil, with the
reverse die being the upper one that was struck. It's been speculated that this was to avoid "striking the emperor".
With the
reverse die being the one that was directly struck, it presumably would have deteriorated faster than the lower
obverse one, with the latter being "cushioned" by the
flan absorbing most of the energy by way of deformation to take the pattern. This would generally explain why there was need for fewer
obverse dies than
reverse ones.
Perhaps specifics of minting practices at different mints and different times affected exactly how much faster the
reverse dies needed replacing, and therefore the ratios observed. Variables like masses of the dies, hammer, anvil, speed of strike, ductility of the
flan, would presumably all have made a difference.
The upper die being the
reverse also explains why
obverse brockages are more common than
reverse ones, since a struck
flan (with
obverse design on it's lower surface) sticking to the (out of sight) upper die would have been less obvious than a
flan struck to the lower die. The upper die taking more of the force of the strike may also have made it more likely for the
flan to be bound more tightly to the die so as not to release.
Ben
Edit: I posted too fast - didn't realize that most of this has already been mentioned.