I was involved with some of the original (don't know what if any changes might have been made since then, after all almost 10 years has passed) Rumble (and Bassman) driver concepts. There were a couple of different Rumble drivers that were considered, perhaps it became clear that it was ultimately less expensive to source the same speaker for all of the combo/cabinet versions even if they may have been more than necessary. The guy ultimately responsible for the finished product did a nice job IMO, the results speak for itself.
That certainly fits with the conservative ratings reputation Fender has. Like the Rumble 200 combo, which can see up to 140 watts continuous, having the same driver as the 115 cab rated at 300 watts continuous. Sure seems like a lot of "giveaway" in the combo. But, I guess, if the economies of manufacturing and assembly are workable, it's better than saving a few bucks and risking your reputation by accidently putting some 150 watt speakers in some 300 watt "rated" cabs.
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