On the java edition of the game when a charged creeper blows up multiple wither skeletons, skeletons, zombies or other creepers, only one of the mobs will drop it's head. On the Bedrock version of the game every mob will drop its head. The behavior for the bedrock version of the game has been marked as WAI. This is a parity issue that should be fixed to create more straightforward gameplay and not require a player to collect large numbers of charged creepers to acquire a few mob heads.