is this legal for an employer to do?

I know someone who works at a facility that has weekend baylor staff.
12 hr weekend shifts. Anyhow, All do the same work and same hrs, but there is a difference on how they are paid and different benefits. Few yrs ago when the old hires were hired, their contract was a flat rate of wage and their paid time off is based on the base flat rate of pay, also all their benefits are based around their wage rate, even when they come in to work an extra 8 hr shift through the week. Ok, now the new hires, doing the same exact job as the old hires, their contract is rewritten as to say that this base flat rate is broken down to a base rate and shift differential and baylor bonus, but it still adds up to the same wage rate as the old hires, but the heres the flip side of it,,under this new contract for the new baylors, if they come in to work a seperate 8 hr shift through the week, they are only getting paid the base rate excluding the baylor bonus and shift differential…as to the old hires under their contract they get paid their entire baylor wage rate, also the paid time off for the new baylor under the new contract their pto is paid on just the base rate out of their baylor rate. and all their benefits are based on just the base rate out of thier baylor rate…it goes something like this..

old 24 hr baylor contract—37.00 hr.base rate for each hr worked..all benefits and short term disability claims will be based on this wage.

new baylor contract–base rate 27.00 hr. + 3.00 weekend shift differential + 7.00 hr baylor bonus =37.00 hr, any shift worked outside baylor hrs will be paid at base rate and all paid time off is accured at base rate, and all benefits and short term disabilty claims are paid at base rate wage…
how can this be legal? especially when they are all doing the same job and the same amount of work..how can two different contracts be active for the same job descriptions. if it is a revised baylor contract, doesnt that do away with the old contract? and if they are saying that this 7.00 hr is only for baylor hrs and it is a bonus then how come it is taxed along with the base rate as a normal wage.