First of all, I believe last season was the first one in which WIS re-did the defensive ratings to their own revised formula. I know because I sent in a ticket when I saw them. There are a TON of useful defencemen in 08-09. Dennis Wideman has a freakin' 95, okay?
Second, here's how I think it works - this is a theory I've had for a long time:
Player + Team defence relative to the rest of the league + PIM + icetime = something like defence rating.
Obviously, players on teams with the best defence each season have the best defence ratings. You will also see that historically the extreme defence ratings show up on teams which far outperformed the worst teams in the league that season. Like the 44-45 Habs, or the Habs of the 70s. Extreme defence ratings actually seem to reflect the presence of terrible teams, in a way. That also looks like why, in the 50s, no players have extreme ratings. The best teams in the 50s gave up 2 goals a game, and the worst gave up 3.
So you take Alex Smith's 93. In 25-26 the Senators gave up 42 goals in 36 games. The worst teams, the Habs and the St. Pats, gave up 108 and 114. Almost 3 times as many goals. Alex Smith was one of only 4 defencemen on the Senators, and the only one who did no scoring.
The 25-26 Sens' D ratings - 95, 95, 95, 95, 95, 93, 91, 90, 89, 85. The last three players, Georges Boucher, Hooley Smith and King Clancy, appear to be docked points only for PIM.