This may be a whacky idea but have you measured the two calipers you have to make sure that they have the same reach?

You mentioned in a post from higher up that you drilled the rear bridge to allow the brakes to mount. On a newer frame that suggests to me that you should have gotten the proper long bolt style so you didn't need to drill the frame. But it also suggests that since you drilled it from the back and right on through that the caliper on the rear may be angled down instead of sitting parallel to the seat stays. If so it would reach further. So it's possible that the rear isn't really long enough either but it worked out due to how you drilled the frame.