As an example:
Why did the germans used 88mm instead of a nice round 90mm? Why was the PTRS chambered in 14,5mm instead of 15mm, why did the germans used 10,5cm howitzers instead of 11cm howitzers? Why did the russians used 76,2mm instead of a round 76mm?
And so on...
Not to discourage other answers, but [here's a similar question I asked last year] (https://www.reddit.com/r/AskHistorians/comments/34qd1a/why_do_guns_have_such_bizarre_calibers/)