It is determined by their pay grade and their length of time in military service. Basic Pay, also known as Base Pay, is the same for all the services. The enlisted pay chart includes pay rates for all grades from E-1 through E-10 and for durations of military service ranging from less than four months to over 40 years.
Perturbation is an anxiety or disturbance, or is a cause of disturbance, or a from what is normal, especially when the deviation is caused by an outside influence.
In particle physics this is codified into a perturbative expansion coming from perturbation theory. Initially the potentials added to a theory were assumed to perturb the free system solutions, thus the nomenclature. It led to the Feynman diagram formulation of cross sections for interacting particles.