You characterize $3-4M difference as "huge increase" but also $3M ($15M-$18M) as "not that far off . . .
First, despite consistent fan predictions/concerns that Edmunds will be paid Leonard, Warner and Wagner money, there is no, zip, nada, evidence that the Bills are considering that salary, no evidence that Edmunds will ask for that kind of money, or that Edmunds is expected to be offered that salary range. Per usual, no one associated with the Bills has said anything, and I've yet to see any non-Bill fan "cap expert" suggest that Edmunds is worth or will be paid like Leonard, Warner and Wagner.
Second, many good football experts say Milano and Edmunds positions are interchangeable. I agree, to a point, but the Bills seem to play to their strengths - Milano as a missile to seek and destroy, while Edmunds is much more a space eater, making the pass game much more difficult. Clearly not everyone agrees, but I think the Bills believe Edmunds' skillset is much harder to replicate than Milano's. The Bills need them both to be an elite defense, but believe the Bills are willing to pay Edmunds more than Milano because Tremaine's replacement would be much, much harder to find, and those worth more to the team.
Not unless terms start with two early first round picks.