because it is counter intuitive. It's easier to believe that gamers have X number of dollars to spend, and that those dollars are divided by Y number of games. So as Y goes up, average rev goes down: simple, believable, and apparently wrong.
That is mostly true if those games have a wide variety of genres and target groups.
If our whole Group is 100 people big and they have a budget between 1 and 5 games they can afford, there is no way every one of the 100 is buying the amount of games they can max afford, ever.
You can have 1 Person who can afford 5 Games this holiday but buys nothing because nothing interests him currently.
Hearts of Iron 4, Star Citizen, Nier Automata, Dark Souls 3 and the Last Guardian could have been released this Holiday Season and there wouldnt be much overlap with the Top 3 Software. Some would not buy Fallout 4 if Hearts of Iron 4 was available, some wouldnt buy Battlefront if last Guardian was available but the majority would just buy the game in the genre that they love who haven't bought anything this holiday season or have not bought the maximum of games that their budget allowed them to.