Please correct me if I'm wrong here.
Some of the ROMs went over to leaked software, some went a step further and require the new boot/kernel.
The first type replaces the system data, and is based on a develepor release. I believe Fission initially went this route, which would explain why Rubix is similar. The ROM only updated your software, and did not update the boot/kernel (which is ok).
Some of the ROMs are now going to require the updated boot/kernel, which means they expect you to upgrade fully, before loading the ROM.
This also happened in Fission/RubiX, because they used parts of the D2 release to create the ROM.
You can determine what bootloader/kernel you're on in the system info, but the build number is really just whatever the dev typed in.
The same confusion happened when Froyo was released. Some people upgraded OTA, and in doing so upgraded their boot/kernel, and some people just used the leaks to upgrade the system data. At that point in time, you generally could run any Froyo ROM without upgrading the boot/kernel, or run the same ROM if you did. If you used a full SBF, however, you had to make sure you didn't try to downgrade the boot/kernel, or you would brick it.
It seems that a split may be happening, where some ROMs will require a complete update, and some will not.
Basically, you need to read the full instructions of the Dev that made the ROM to be sure, and if you aren't sure about what to do, wait until a full SBF for the new leak is available.
/Better to be safe, than brick.
//If you're happy enough with what you have running, just hang in there, until a proper SBF is released. Until then, enjoy your smartphone. It's not as if the screen will get any larger by updating to something you aren't sure about. Fission is running great for me. RubiX blurry is a great ROM, and there are versions that don't require updating the boot/kernel. ApeX has great reviews, and ran very well for me. Tranquility 3.0 runs just fine. SSX ran great for me. There are tons to pick from, without the need to update without needing the new kernel/bootloader.
///I really want to try Jrummy's ROM, and Darkslide, but I'm going to hold off for a proper SBF to be released.