[sokoloff]

With overlapping bands, it’s not necessarily the case that you were being underpaid (in a cosmic fairness sense).

If you are a level N Foo and doing well (as determined by doing some amount of level N+1 work), you might be earning the top of the level N Foo band. If that’s above the bottom of the N+1 band, you might be making the same as or more than the most recent N+1 hire.

The day the company decides that your level “rounds” from N to N+1, you might already be being paid the exact correct amount.

Design of pay scales and how they interact with promotion thresholds is complex enough that many fair systems can evolve. Most systems have evolved to slightly underpay the “almost N+1” employees for a period of time, leading to the expectation that promotion come with an immediate salary bump (and often companies do that to make the promotions “feel bigger”), so maybe the problem is promotions under that system should come with back pay rather than the situation described here being the one where back pay is owed.