Anyone who counts Henderson as the #2 all-time middleweight is including his achievements outside of the 185-pound division. You can't do that, that doesn't make sense. He knocked out Fedor as a heavyweight, knocked out Wandy for the Pride title as a light heavyweight, beat Vitor at 205, won the SF title at 205, had his all-time great classics against Shogun also at 205.
As a middleweight, his achievements include: UFC 17 tourney winner (Although he weighed 193, pre-unified Rules), 2005 Pride WW GP winner plus the title he never defended, the Bisping KO, and knockouts of Boetsch and Lombard of note.
Compare that to izzy, my gosh, it's not even close.