If I understand the Binance situation correctly they do not claim to be holding USDC for you, if you give them USDC then they convert it to their BUSD token (also a stable coin) and that's what's listed on your account. So when you withdraw your BUSD to USDC, they may have to go buy some USDC to actual service your withdrawal.

I don't use them, I'm going off of this page: https://www.binance.com/en/support/faq/what-is-busd-auto-con...