When you summon a Zombie with the NBT Tag "DrownedConversionTime:1" it summons a Drowned Zombie, which is how it should work.

But when you summon a Zombie Villager or Zombie Pigmen with this NBT Tag it still summons a Drowned, every other mob this does not happen, including Husks (it summons a zombie because it goes to zombie than drowned) and Zombie Horses. 

Zombie Villagers and Zombie Pigmen do not drown so it does not make sense why it does with commands.