Factory firmware is the routerboot version installed on a device at manufacturing time. There's nothing special about that and is usually the version available in ROS at the same time.
Just an illustration: I've got a hAPac lite and a hAP ac lite TC. They are the same hardware just in different cases (the TC actually doesn't mention it in software, so without physical contact you can't tell them apart). One has factory routerboot version 3.33, the other one 3.36. They both have routerboot version 6.43.something now.
You can see the factory routerboot version mentioned and the only meaning that information has is that you can't install routerboot older than factory version ... no matter what. And with a reason: it seems to me, but I may be wrong, that there is single routerboot image per platform and later devices might come with new peripherial hardware (e.g. wifi card) not supported previously. If one could install routerboot predating support for such hardware, it would become unusable (or, in worst case, device bricked).