Bolt Pattern (stupid but gotta ask)

[Del72Chevy]

I know most of you will think this is a stupid question but I have to ask. I have a 72 short bed and I was under the impression that the bolt pattern for the 5 lug was 5 on 5" I just happened to put a stick rule on the studs on my rear axle and center of stud to center of stud is 4.75". I know you have to measure across the center to the other stud and not to the stud right next to the other. I know that much but why am I getting 4.75 and not 5". I was curious and grabbed one of my front rotors and it measured the same 4.75". Am I goofy or what?