I understand why it shouldn't be 6, and i'm fine with 5, but where does 5.25 come from?

As the (now-former) owner of a Blood Botkiller Medi-Gun (see last link in your proof), I can attest to the challenge in trying to successfully sell for 6 keys. I ended up settling for a cheap-painted Bills for the Medi-Gun + 2 keys + 0.66. Woe is me... D:

That being said though, I agree with Oreos. That one trade for 5.25 keys happened only because the seller (who wanted 6 keys) was desperate enough to negotiate with the buyer, who wanted to purchase it for 5 keys. Because of the fact that the trade never materialized (the buyer ended up buying elsewhere), I don't think that 5.25 keys should be considered. What do you think?