One of the problem with "standards" is how they are articulated between companies.
A 1.5" post is "more or less" that diameter, held to certain tolerances set by each company to work with their own gimbals and their tolerances. If, for instance, if a post is nominally less than 1.500 with a tolerance that is such that the post never exceeds 1.500, that is one way to describe a 1.5" post.
Another company may set the nominal at 1.500 +/- several thousandths, and their gimbal will be able to clamp to the largest acceptable diameter when the gimbal tolerance is at its own minimum acceptable diameter. And there are other tolerances for roundness, straightness, that might come into play.
Swapping parts between brands might get you a situation where the tolerances overlap and all is okay, or you are at the extremes of each part and they won't mate. .005 inches is the difference between a very loose and a very tight fit at these diameters.
Everyone should have a set of good calipers and know how to use them, especially if they are into making their own "Frankenrigs."