To be honest it's just a thing from financial models inside industry. When you plan how much copies you suppose to sell to turn profit you must consider refund rate as well.
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.
