You are not crazy. It seems reasonable to expect that the rulers would "measure" identically. They don't.
I just threw away THREE Quilter's Rule rulers (two 6-1/2 x 24 and one 4-1/2 x 14 inch one) because they were "skimpy" by more than 1/16 inch in the center of all of them. I don't know if they were worn down from use or "off" to begin with.
Do you have any idea how hard that was to trash those rulers? But they were "off"