Interesting. However, I wouldn't say my measurement is exact as I wa just using a tape measure but with yours being slightly further out then cases such as mine would be even more centered. Time to ride ::001::
Couldnt begin ta say why theyre different than the fixed racks....but Ive added a couple of washers to each top bolt on the right side to bring them closer to matching the left width. More washers or a spacer would require longer bolts & tilt the top of bag out a bit more. My boxes are now within 1" width of each other from bike