OK. That makes a lot of sense.. but now I don't understand why after moving to say 246 you can't FXZ back. I assume moto is doing something to the firmware to say you can't use that full system image anymore? Just curious to understand the whole thing. Sorry if it sounds like a stupid question.
You need to understand what a locked bootloader means. Ever wonder why we can't just flash any old thing we want? That's the security of a locked bootloader. For the builds 235 and above, they upped the security on the partitions. If you try to flash and old image for a partition, you will either fail to flash or brick your phone. This is why you can't simply FXZ back to 905 from 235+. The security of all the builds 235 and above is the same, so you can move between those builds without issue. But no FXZ'ing with 905.