People watching the leaked WM builds that have noticed that the numbers vary wildly and sometimes a newer build comes with a lower number. Herg stumbled upon the answer on XDA and this explains the build numbers:
212xx = AKU1, all builds leading up to and including WM 6.5
213xx = MOT Motorola
214xx = ????
215xx = SAM Samsung
216xx = HTC
217xx = COM1, continuing dev of 188.8.131.52 – 184.108.40.206
218xx = COM2, continuing dev of 220.127.116.11
219xx = MD, feature branch (unstable features are added here, backported to 218xx – this tree is based on COM1, so older base os code, but the UX code is newer [user experience])
22xxx = SEMC Sony Ericson
230xx = COM3, continuing development of 18.104.22.168
234xx = COM4, continuing development of 6.5.1
236xx = LG Electronics
Most new builds are 230xx or 234xx and here’s where you can see the difference. In that 230xx line you can expect builds that are very stable because these are official builds whereas the 234xx builds are going to contain the newest features that are experimental to some degree as this line is the likely precursor to a WM7 build. So this is why some chefs are only building off of one build and why Herg is now building separate ROMs depending on which build you would want.