a) why the different firmwares?
Because Samsung feels like it ... they were probably meant for different OEM markets ...
b)what is the difference(s)
c)is one drive better at reading or writing than the other?
I doubt there are any differences between firmwares with the same base / date stamp.
SB02 should be better than SA00 because is much newer and might contain fixes, better media support and improvements in the writing strategies.
d)i cant cross flash the sa00 drive with sb00 or 02 firmware or vice versa, why?
You can't flash a drive with a firmware with a different OEM code ... even though the hardware is the same, the Samsung flasher checks for an OEM code match, and it won't let you do it. It is an intentional flasher limitation.
However, you can crossflash between OEM codes easily, but telling the flasher to behave ... i.e. running it with the -nocheck option.