Are you still running it at 35 psi, too? If so, it sounds like the 1.75 tire is deforming enough that its rolling circumference is closer to a fully-inflated 1" tire. But, the front tire wouldn't deform nearly as much as the rear.

Sigma might have also programmed its 26x1.75 setting as if the bike had knobby tires instead of smooth tread. The tire size is more about its carcass and the room it has inside, not how deep the tread is, so a smooth 26x1.75 slick tire would have a smaller "on-the-road" circumference than a 26x1.75 deep-lugged mud tire.