Not sure,,, a mm or two doesn't seem too big of a deal.
As a collector, I do not measure mine - and some I've received were a little under, that didn't bother me really, but it might bother some collectors.
I for the most part just buy mine from Strathmore - since they offer them in a variety of surfaces and papers, and they are EXACT - so I just do not have to even remotely worry about it for my originals anymore.
They run me like 2.65 a package - so you know I just buy like 10 bucks worth at a time, and I'm SET for a good while. LOL.
Here is a link to the cards sold at Dick Blick:
www.dickblick.com/prod...ing-cards/