"I didn't really understand why they would make two sizes for one caliber anyway. "

They don't. What has happened is that a various barrel makers make a "50 caliber" barrel, but some are as tight as .495" and some are as loose as .51" or more bore diameter, and groove depths vary even more. For example, I have a custom .50 barrel that uses .500" balls with a .010 patch, and this combo's not really too tight. The same applies to most ML "calibers"