[BrianG]

Think of it like this: Pretend there is a plain black box in front of you which contains a battery of unknown size and just has a simple set of terminals. You are only told it is a Li battery and given a voltmeter and you have to determine the cell count. So, you measure the voltage, and then based on assumptions about lithium packs (like a healthy cell voltage can vary between 3.0v/cell at near dead to 4.2v fresh off the charger) you make an educated guess of the number of cells. This is basically what an ESC is doing, except an ESC will always assume it is connected to a fresh pack when it is powered up. It doesn't know if you are trying to trick it by hooking a depleted 6s pack, it will just see it as a fresh 5s pack.

And like Serum says, the voltage of a near dead pack will slowly rise back up a little when the load is removed for a while, and may just hit the threshold of what the ESC determines as 6s.