The general reason for these odd numbers is that they are conversions from before the metric system. 37mm is about 1.5 inches, 76mm is about 3 inches, 88mm is about 3.5 inches, 152mm is about 6 inches. Early tank and anti-tank guns are particularly prone because many were adopted from naval guns like the [US WWII 76mm] (http://en.wikipedia.org/wiki/3-inch_Gun_M1918). However, every country has a different specific reason for specific weapons keeping the old non-metric calibers, and I don't know enough to explain why the Soviets or Germans, for example, kept the odd calibers.

I'm unclear about the question. Of the two examples you give, two of them are even numbers. Only 37 is odd. And the examples you give of even numbers include one odd number...

As near as I can tell, what you mean by "odd" is number that are strange, and not rounded off to the nearest five or ten? Is that correct?