Ahh yes, the age old question. Basically in the USA we decided that 5/8 was the size a mic stand thread should be, and over in Europe, they felt that 3/8 was the answer. And they both work - and as you noted, there is usually always an adapter in case you get into a situation where it isn't the size you need.
In fact, every time I get a new mic it has one of those adapters with it. Or maybe they're coming with the mic stands. Heck I don't even know, now that I think of it. But I have more than a few of those things laying around!