I'm surprised the SF Banner has stayed at around 9 keys
i've never seen that many people use the regular one for some reason, and the other f. stranges are more widely used and so are in higher demand
this is why the s. f. medigun (most people use medigun), is worth way more than the s. f. flamethrower (which many people don't use)
this is my interpretation and i'm pretty tired so idk