You can get Tullius armor easily when you side with the stormcloaks and do all their quests...
I know, however I'm willing to bet it's actually a duplicate item that's part of his 'death item' list, which governs what gets dropped when something/one dies, since otherwise you'd be able to steal the one he's wearing while alive. They do this for weapons, too; NPC archers use an unplayable version of each bow that doesn't use ammunition, which is why they're never out of ammo when you loot them no matter how many shots they fired.