This has nothing to do with Roman numerals. The operating system commonly called MacOS 9 is actually the ninth version of the operating system named MacOS. Except for the QuickTime frameworks, the Carbon frameworks, and the Classic environment, MacOS X had almost nothing to do with MacOS. It is the current name of the OS formerly known as OpenSTEP and NeXTstep before that.
MacOS X began its new life as MacOS X 10.0. However, it was not the tenth version of anything. Each MacOS X point release has been a major evolutionary step that would have merited a new whole version number in other versioning schemes. For example, MacOS X 10.5 brought certified UNIX 03 to the Mac platform. MacOS X 10.7 will bring many UI elements from sibling iOS. Apple has the option and the right to name its OS anything that it wants. When it chooses to give its OS a new whole version number, it will be perfectly within its rights to call the OS MacOS X 11.0. I'd bet dollars to donuts that we will never see MacOS XI.