Most likely it's a fuse type thing. It literally is a one time programmable fuse that is set by some subroutine. Basically you execute that function and bam you get back some key that says that it worked. You try to execute again and the fuses have already been set and the key is the same. Many ICs have built in one time programmable memories or fuses to be used during provisioning. Such as to set a MAC address on a Ethernet chip or wifi chip. It's literally burned into the chip in the factory and cannot be changed. It's a one time thing and does not change after reboot or anything.