And I like to think of it the other way around: if they're offering you more money now, why didn't they do it sooner?

Either they were keeping money you deserved for themselves or they're just giving it to you now so you don't go away because they really really need you.

In the first case, you'd be better off without that sort of people. In the second one, you might be sacked as soon as they don't need you anymore (of course there's always the possibility that they'll need you forever and ever...)