I really don't know, but maybe there were some small physical changes in the different board revisions (otherwise - why different revisions at all?) that made newer firmware versions ever so slightly incompatible with older board revisions? The firmware works very tightly with the underlying HW and maybe the reason to why there weren't a publicly released general firmware update that would work on *all* boards simply because it wasn't worthwhile to spend the time and resources to do this? Just thinking out loud...