Well, I based my version of the smoke rules on Simon Walker's, who is active on the Colonials board, so if you want you can ask him too...
My own impression/assumption is that the idea is that it's meant to also account for how, with less time between firing, the smoke also has less time to disperse in between and so will be more concentrated? A muzzleloader firing once every 20 seconds and a breechloader firing once every 10 seconds will produce the same amount of smoke per shot, but the breechloader will produce twice as much smoke for a given period of time. It's therefore not so much that the higher smoke rate reflects a higher rate of fire, but more that the higher rate of fire will produce more smoke faster.