How much of a cut does the publisher and steam take from a $20 purchase?
* Refunds and VAT 16% overall
* Steam cut 30% below $10,000,000 sales
* Publisher cut from net profit after recouperation is around 50%
So developer get around $5.88, but obviously for some countries it's can be better because there is no VAT or lower VAT, but price there would be like $5-15.
PS: And yeah from $5.88 you still owe some corporate tax.
Another reason is that games on Steam can and are often refunded after quite long time has passed. E.g people buy lots of games on holidays or sales and then refund them when they decide not to play it. Sometimes refunds can happen even after a year if e.g player can't get your game to run on their device, etc.
But yeah it make sense not to include it in math for single sale that was not refunded.
As for refunds AFAIK if you have bunch of refunds later they will just be substracted from future sales. But games I worked on didnt have refund spikes so no clue how Valve handle situations with mass refunds.