[lowercased]

> Current company knows his worth

Or... they think they do. "Current company" is too amorphous. Direct manager and team mates may know value, but it's harder to make that judgement (or to believe someone making the case for it) when you're not in the trenches with that team.

May also be a situation where the new company is actually able to extract more value from the person than the previous company.

There's a lot of assumptions going on in some of the replies here. Old company is going down the pan - regardless of how much value an individual can bring, if the company is incapable of translating that in to market value, you'll never get the raise (and may not have a job soon). New company - taking the same person with the same skills - may be able to extract much more market value from that same person.