I've got a feeling that the 11 key sale was actually a quicksell.

Kostek often quickbuys items off marketplace.tf, as you can see in the item history ( https://backpack.tf/item/3752114906 ) it left the seller's backpack on the 9th of April. The sale on marketplace.tf also occurred on the 9th of April. The marketplace.tf bot doesn't even appear in the item history which is an indication of how quickly it sold.

Basically it looks like the 11 key sale literally just took a few hours ( https://gyazo.com/1feb91422d8785fc2b7b47cfc9892df6 ) whereas the 15 key sale took 3 weeks.

I personally feel like 15 flat would be better here.