Pica ruler and Inch side not accurate?

[7on]

Is there a difference between onscreen picas and analog picas? I have a ruler with 6&12PT on one side and inches on the other. The problem I'm seeing is 6p0 lines up with 1inch - but at the opposite end it look like 12inches does not line up with 72p0 (I would guess the pica side being 2-3 points short...) Is this just my eyes playing tricks or is the ruler just wack? It is a Gaebel ruler.

Thanks!

 

[TEG]

It is more than likely a problem with the ruler. I do believe there is a difference when dealing with PostScript, 72pt to an inch vs 70pt to an inch.

TEG

 

[7on]

It is more than likely a problem with the ruler. I do believe there is a difference when dealing with PostScript, 72pt to an inch vs 70pt to an inch.

TEG

the weird thing is that it's only 2-3pt off on the same ruler.

I've always just guessed (if it's close to 39p0, then it's 39p0) but that's not always good, and it has come to bite me one or two times.