You are over (or perhaps under) thinking it. There is only one explanation: he's worth $7.5m because someone is willing to pay him $7.5m per year. Period.
Any comparisons to other players, their stats, salaries, etc., is just noise that misses the point entirely.
GO SABRES!!!
"Top dollar" is a constantly moving target. Compared to others, he's not making close to top dollar; he's closer to "middling dollar."
As contracts continue to escalate in value, his deal will look more and more middling, too.
GO SABRES!!!