With the longer TT you use a shorter stem which puts more of your weight over the center of the wheel base. In theory that is suppose to help dampen some of the road shock. It also makes the steering feel a bit lighter which is a plus when you have loaded panniers on the front. This was/is common on French touring bikes which the Riv is loosely based on.

BTW nearly a pound of the weight difference is in the saddles. You will want the Brooks eventually