What you are experiencing has to do with the dynamic range of a sensor. There is a limit to how much range a sensor can actually pick up. The change in light from brightest value it can read to the noise floor of the sensor determines what the usable dynamic range is. Image processing for a standard exposure tends to try to use a fairly fixed subset of that range. If the sensor has a much larger range, then further recovery can be done, but if it is only a little bit larger, then not so much recovery can be done.

It can also vary a bit from shot to shot as some shots may be taken more to the dark side of the dynamic range and some may be taken more to the bright depending on how the scene is lit. You'll still always have the entire dynamic range to work with, but the information you want may be captured on the brighter side or the darker side. This is why you sometimes here things like expose to the left or expose to the right for either limiting noise or highlights. *Underexposing limits highlights but increases noise, over-exposing increases highlights but decreases noise as darks are further from the noise floor.)