Cash app is basically just an exchange so I seriously doubt they have dedicated accounts on the backend. Their support might be able to give you a more direct answer
Discussion
Actually from their support page they actually make it clear that standard withdrawals are free. I would imagine they are batching transactions on the backend when they can to keep costs down.
https://cash.app/help/3103-bitcoin-pricing
The more annoying thing is their withdrawal limits, which are worth looking at and keeping in mind.
Appreciate it!