[ojars]

Re-syncing is not the same as re-programming.

Some one more knowledgeable may have a better answer, but I offer this --

When you press the appropriate button on the key fob an RF signal is transmitted, which the alarm unit will sense if in range of the signal. The content of the signal or the signal frequency must be recognizable to the alarm unit before its computations are triggered. Re-synchronization of the transmitters is a protocol that allows that to happen.

When you must use the PIN and re-program the fobs and the alarm unit, the process gives all pieces the appropriate content for transmission.

These are my speculations; do not mistake these ruminations for expertise!