Outside of HR pay band constraints, I’ve often wondered if there’s some psychological anchoring at play [1]. No matter how many more duties you take on, your monetary value may be heavily anchored by what you were previously paid.

However, a new employer doesn’t have access to this information so they can make a more objective determination of worth without being biased by this pay anchoring aspect.

[1] https://en.m.wikipedia.org/wiki/Anchoring_(cognitive_bias)