If anything, Epic is showing that, at minimum, 25% is required to operate a store with basic features. For 12%, Epic does not cover payment processing fees and instead passes them along to the buyers on their storefront - they're keeping a hard 12 for themselves and nothing else. Apple, Google, et al, eat the payment processing fees which can range from 5-25% of the transaction depending on where you are in the world (in other words, a 60 dollar game can become 75 in some markets). If a developer wants Epic to absorb those costs, Epic charges a 25% fee.
Out of their own mouths…
And what Valve says about those transactions:
Beyond the fact that this is incredibly worse for the consumer... it's also worse for the developer. They now need accountants to manage the income and what they need to pay to Epic. That's right, Apple pays out and provides tax documentation. Epic sends you the full money, and an invoice you need to pay and zero tax documentation. So what does Epic do for 12%?