When using transparency mode the headphones that you are using elevate external sounds and play them directly to the users ears.
This enables the user to focus on and be more alert to those sounds. This can be extremely helpful if you are having a conversation with someone or you’re on a busy street with lots of cars, just to name a few examples.
While I’m sure it’s possible to still ignore certain sounds and frequencies, it’s not unusual for sounds that you yourself are making to be picked up by the headphones.