I can't give a definite answer but sometimes its a simple or minor difference yet could be major but difference is small. To explain that is that sometimes it may have fewer ports like USB where its reduced by NOT installing(during manufacture) the header on mtrbd., because for whatever reason it becomes that model. At the same time, that model may progress later offering the added ports. The same could be said of SATA ports, many times in older mtrbds. the actual mounting area is present but missing the physical connection. Later the same mtrnd. is offered with all ports present and ready to include any bios updates required. This is very prominent in OEM offered PC's as the difference from one model is slight from the next to include later offering. Some PC's become obsolete yet replaced by a new model that basically is the same but different in only one area as explained above. Another example would be the older AGP video buss vs. the newer PCIe-16x.
I have gone to HP Compaq site to try and see what the difference is between 2 motherboards. Spec are same same chipset. What is the difference between NodusM GL6 MB and the Nodus M3 GL6? I can not find a answer.