The International Organization for Standardization (that body responsible for the various ISO quality standards) and every dictionary I can find defines a ream (of paper) as consisting of 500 sheets. This is is good: it confirms certain experience gained in my former life as a federal purchasing agent.
In preparing to order printer paper from a well-known company whose catchphrase is “That was easy” (formerly “Yeah, we’ve got that”), I see that most, if not all, paper they sell is described as “400 sheets/ream.” My guess is that someone is not aware of the definition and is just assuming that a package of paper is called a ream.
A foot is twelve inches, a pound is 16 ounces. If one was selling products by those measures but providing a smaller quantity, they wouldn’t get away with it. Or would they?
Question: Is there a law or regulation (you know, one of those things we have to enact to enforce common sense?) that says if you use a standard term in selling a quantity of a given product, you have to abide by the commonly accepted dictionary definition of that term? Can Staples (OOPS!) sell 400 sheets of paper and call it a ream?