I can only make a S.W.A.G. about the scallops, but it appears to me that these have the effect of slightly slowing the movement of the slide when there is one or more rounds of ammo in a loaded mag in the gun. These short barreled semi autos have a pretty narrow window of tolerances to allow them to operate with proper slide speed and force for reliable operation. The complete recoil and reloading back to battery operation can also result in battering of the various parts of the mechanism that stop both the forward and rearward motion during the cycling of the slide after firing.
My copy has been entirely reliable from the beginning, but I expect to probably change the recoil spring assembly a bit earlier than I will with my full sized M&P's. I suspect a weakened (by use) recoil spring will negatively effect function in these short barreled guns.
As noted, this is just speculation on my part, but I feel certain that those scallops in that location did not get there either by accident or a simple decision to save the bit of time required to smooth them up in production. Since my 9MM Shield does not have the scallops, I believe they are present by design, and my reasoning is the only thing I can think of to make this necessary. My two cents FWIW!