As far as I can tell, there’s two aspects to this:
1. Smart contracts
Going between Incognito and the mainnet involves the interaction of smart contracts. This is greater computation, and thus greater cost than a simple transition of ETH from one wallet address to another. This video explains the mechanics.
This thread actually inquires about how many gas units it generally takes to unshield to the Ethereum network. (A standard sending of ETH is 21,000 units of gas.) The response there is that to create unshield transaction it will need ~260,000 gas for ETH and ~280,000 gas for ERC-20 (again, because of the involvement of smart contracts.)
That’s one part of the reason for the cost discrepancy. (For more on that, search the forum for something like “unshield fee”)
Another part is:
2. Incognito fee
Apparently Incognito charges a fee that is hidden in the displayed network fee…
Years ago someone created this thread asking about fees, and it was originally said that “Incognito does not earn any fees when you unshield.” But years later someone else bumped that thread, asking if that was still accurate, because they noticed the fee changed based on the amount they were trying to unshield.
Someone replied that “0.3% is taken as unshielding fee to compensate unification cost of multi-chain assets such as USDT, ETH etc.” Personally I’ve never seen this disclosed anywhere except in this forum. And it’s definitely not made clear during the unshielding process.
The lack of transparency on this is disturbing enough, but it also doesn’t add up, as I noticed the fee being applied to XMR (which isn’t involved in unification).
And also, if you notice the latest posts in the original thread, someone also reported the fee being seemingly included in BTC unshielding as well.
I would be very interested in some clarification on this.