Expected: I think it's intended that Lazarus is the complete stockist who sells every Mage scroll in the game.
Observed: actually he stocks everything except "Protection from Petrification". Since that spell isn't likely to be needed at that stage in the game, I doubt that this is a deliberate omission to make the player go seeking it elsewhere ... most likely it's just a mistake.
Note: I know this is pretty trivial, and it won't really matter if it's not fixed. Nevertheless, for the sake of completeness, maybe one day someone will get around to fixing the omission.