UNCTUOUS
I don't think they use battery backed RAM, rather I think they use a type of memory called EEPROM (E2ROM, - electrically erasable read only memory) which is inherently non-volatile (retains memory without power applied). However this type of memory has limited write cycle capability as you have seen. It is something to do with electric field stress during programming. This value is typical for an EEPROM device. Incidentally there is a similar type usually just called Flash used to hold microcontroller program code which is rated at only about a thousand cycles as it is not expected to have to be rewritten so often.
FujiF