This seems to be a matter of manufacturing and in some cases quality (in the sense of You get what you paid for) which is more speaking of the choice of wood, type and thickness of core and type and thickness of the veneer. The one thing you didn't explicitly imply either is that the thickness of a sheet of plywood could also be inconsistently think throughout the sheet, which is also a pain. To quote the article that Maxime Morin found which also concurs with bowlturners comment:
The reasoning is that an undersized thinner panel will still fit in a groove cut at the nominal size, but an oversized panel would not.
It is hard to predict what each sheet class of plywood will end up like post manufacturing which is why there are grading systems1. It is also important to keep in mind that moisture will play a small part in this ( again.. more deals with quality.)
Looking at forums from people discussing this issue is just accepted as fact that this is a possibility that needs to be accounted for. One of the many reasons why I own digital caliper. Tool manufacturers are also aware of this because you see products like undersized plywood router bits which are
Designed to cut slop-free dadoes for today's slightly thinner sheet goods
Quoted from product description for Freud 89-650 Four Piece Undersized Plywood Router Bit Set
This is one reason why MDF has such a cult following. It is easier to get a more consistent thickness and appearance (no knots!). MDF is recommended when you need to have consistent dimensions and don't mind the extra weight.
I'm trying to see if there is any worthy technical reference articles that discuss this variance but have not found any as of yet.
I can't find it again but there was a comment from another WW.SE user who mentioned something along the lines of "This is why measuring in the field is important". I believe the point being made was don't assume measurements. Measure twice, cut once.
1. Grading systems vary by region and can vary between manufacturers. The sentence is included to bolster the point of not every sheet is created equal.