a) the company is growing by leaps and bounds and would rather promote internally X% of existing employees.
--or-
b) the company is facing a downturn and would like to get rid of X% of existing employees.
In neither case does it have much to do with the actual people falling into or out of that X% (unless a perfectly just system is in place [and it isn't]).
Most firings I've seen have much more to do with the people involved (harrasment, incompetence, interpersonal problems) than with wild changes in the company's trajectory, though I will admit that at early-stage companies this could be reversed since things are much more uncertain.
a) the company is growing by leaps and bounds and would rather promote internally X% of existing employees. --or- b) the company is facing a downturn and would like to get rid of X% of existing employees.
In neither case does it have much to do with the actual people falling into or out of that X% (unless a perfectly just system is in place [and it isn't]).