perhaps we can explain the behavior this way. tell me what you think racerx.
the darwin kernel has been changing numbers since early Rahpsody/ maybe even OPENSTEP days. it had different codenames (ie, not darwin, but feynmann, milnor, euclid, riemann), but its version number has been increasing steadily.
apple releases a darwin distribution (ie, a functional OS, rather than just the darwin kernel), and numbers it starting from the OSX PB. so darwin distribution corresponding to OSX PB is darwin 1.0, but it uses the apple version of the mach kernel 5.x (perhaps the kernel numbering starts at 3.0, since that was the last number assigned by CMU). the darwin distro corresponding to OSX 10.1.4 is darwin kernel version 5.4. but the distribution number corresponding to OSX 10.1 is only 1.4.1. this is because it has only been counting since the public beta, while the kernel has been counting since the early days of NeXT.
so if we looked back at earlier versions of rhapsody or OSXserver, or OpenStep/NeXTSTEP, we might find kernel versions back to 3.0, or maybe even 1.0. presumably they would not be named after darwin, but other scientists.
perhaps this habit of codenaming the kernel versions after famous scientists started at CMU and we could find mach 1.0 codenames einstein or some such.
this is all conjecture, so tell me what you think. and i don t think anyone would be in a better conjecture to answer this than you, racerx.