That there's no relationship AT ALL between the OS's?
While OS X is based heavily on NeXTSTEP (and most developer API class names on the Mac are prefixed "NS"), I wouldn't go so far as to say there is no relationship between the "classic" Mac OS and OS X. OS X's standard filesystem is HFS+, which was released in 1998 with Mac OS 8.1, and which shares the same format as its predecessor, HFS. And decisions and limitations from those days still unfortunately put their marks on OS X. For instance, the Labels feature from Mac OS which was bolted back onto OS X (after much public outcry) are still stored in the same place on the filesystem, and in the same format (bit fields), as they were in 1988! And the new tagging feature introduced in Mavericks, for the sake of backwards compatibility with Labels, uses this same area and format to record Tag information! And of that, only three bits are available for storing color information on HFS+. This is why Labels-cum-Tags are limited to the same seven damn colors Mac OS had when Ronald Reagan was still president of the USA.