10" rear backing plates??

OK, I was just curious as to what prevents the 1 3/4" backing plates from not working on the 2 1/2" brakes when they are both 10"?? I guess I am always intrigued by this stuff. Just curious? Do you know what exactly prevents them from working?

Lay one each of them side by side on a flat surface and take a tape measure to measure the offset.