Why does my second command summon 4 baby zombies instead of just one?

I want to know why my first command summon 1 baby zombie and my second command summon 4 baby zombies.

First command:

summon falling_block ~ ~1 ~ {BlockState:{Name:activator_rail},Time:1,Passengers:[{id:command_block_minecart,Command:"summon zombie ~ ~ ~ {IsBaby:1}"},{id:command_block_minecart,Command:"kill @e[type=command_block_minecart]"}]}

Second command:

summon falling_block ~ ~1 ~ {BlockState:{Name:activator_rail},Time:1,Passengers:[{id:command_block_minecart,Command:"summon zombie ~ ~ ~ {IsBaby:1}"}]}