That whole "round up/round down" naming of calibers has often made me scratch my head too. However, it's not only limited to the gun industry, the automotive industry has been doing that for years...so has the motorcycle industry. I'm sure there's a bunch of other industries who do that, too.
Example, a Ford 302 is actually 300.8 cubic inches (or something close, I forget)
A Yamaha 1200 V-max is really 1198 Cubic Centimeters.
I think they name them something close, merely to coin a "catchy name" from a marketing standpoint.