At the end of the day the retention department have a budget which they can not exceed. The individual CS staff will offer deals when they can and I suspect they are overseen by supervisors to make sure they don't give deals to friends and families (which has happended).
There always comes a time when it's more expensive to keep a customer then to let them go
The odds are that if you do cancel for whatever reason then you will get a call back at a later date with an offer even if you have turned down or not offered a deal when you called.
__________________