Maybe it has something to do with players who play the game often making multiple accounts and buying stuff on them to make themselves richer in game... Because that's what I did. Bought 4 keys on 2 free accounts.
Pedro The Hutt said:
But... considering these days you can't use external programs anymore to idle while not in-game, that means they'd need to have five PCs (or more) capable of running TF2 in their possession, surely that would indicate that they're already in a fine financial situation. =p
And I would certainly hope that these guys don't make up the bulk of that 30% that goes from a F2P to a full TF2 account.
Well you only find a random number of items for 10 hours a week, so you really don't need multiple PCs. You just idle for the 10 hours and then open up the next account. I actually do have 2 PCs in my household that have TF2 installed on them, but I rarely need to use the second one because my friends use me as a middle man for their accounts anyway so I usually just have them help me out.
It's also very possible that they do make up a bulk of the percentage as I said, I made 2 free accounts and bumped them up to premium. As did a friend of mine.