It depends a lot on the union and a lot on the culture of the union (and a lot of that culture is largely dictated by country), and on the agreement with the em

It depends a lot on the union and a lot on the culture of the union (and a lot of that culture is largely dictated by country), and on the agreement with the employer. There's plenty of unionised workplaces (especially when it comes to professionals) that have large variation in pay, and there's other examples where pay is based on some formula with no variation (this is vastly more common when it comes to labourers and tradesmen).