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I'm not sure I follow you here. What do you mean they take a "slightly smaller fee compared to a transaction's fee"? They are charging 1772 sat for a withdrawal in 6 hours. Given that even without batching a withdrawal would be a simple 1-input 2-output transaction of around 220 bytes, 220 sat would probably be enough to confirm within 6 hours. Almost all centralized services overcharge for withdrawals because they have to cover the costs of consolidation transactions which happen in the background, as well as take a bit of profit.
they always use the "high priority" fee not the minimum amount. which means if the transaction size were 220 bytes then they have to currently pay 8800 satoshi (40 satoshi/byte according to
https://jochen-hoenicke.de/queue/#0,24h) which makes the current withdrawal fee of 2251 satoshi a lot smaller than what you would have paid for an individual transaction. keep in mind this is the slow (batched) withdrawal. there is another option to withdraw fast and in one tx without batching and that charges the correct individual tx fee value for a high priority tx (close to value above around 9000).