Actually, the Kubel loses damage from closing in on it, the Kubel fires shorter bursts at close range. At long range the Kubel has the burst advantage, firing longer bursts. At close range the Kubel fires shorter bursts than the M3, and gets wrecked for it. A long range fight is RNG, but a close range fight should go to the M3.
Had to actually run the numbers, and well...your point is the correct one.
Kubel DPS at 1 pene with 100% accu: 47/41/39
M3 DPS at 1 pene with 100% accu: 51/35/25
So while the Kubel doesn't really give a shit what range it is at against the M3 DPS wise, the M3 very much wants to close in because it's long range performance is so bad relatively.
M3 < Kubel at long range, M3 = Kubel at midrange (total RNG fight), and M3 > Kubel at point blank. So knowing that the M3 has to rush in to actually win, you can just deal with it accordingly.