Teachers Levelling

On the teachers strike, what exactly is a "relevelling" exercise? I recall having 1,000 employees. I paid increments on a performance basis. It rewards the best productive workers. Great for incentive and morale. It worked well for 2 or 3 years. After a few years - a new HR guy said we need to "relevel" all rates due to years of service. Of course - no one's rate went down via this exercise. So those who had been around 20 years and were not performing - all of a sudden were paid more than better performing younger colleagues. Thus ruining productivity.