villager not being turned into zombies 100% of the time

I allowed 20 good villagers to be infected by a baby zombie. Despite being on hard mode exclusively, 3 of them actually became zombie villagers and 17 just inexplicably died. I can't imagine how on average, 15% of villagers turned into zombies when it should be 100%, given the difficulty. 15% is lower then the percent chance it would be if the game was on easy mode and yet its on hard mode. How could this have ever happened?