It doesn't matter, it's been confirmed multiple times by now.

I don't know the mods exact reasoning behind this but I could imagine this:

Aussies have many available and if you want to buy an aussie you pay a little bit more to get the killstreak you like, however that doesn't apply to unusual weapons as there are often few available that have the wear, skin and effect you desire so the buyer doesn't pay more for the killstreak as it is often the only one on the market.

Simular to painted hats, if a hat is painted people that like that hat and would've painted it anyways buy the already painted hat and pay a little less than buying the hat and the paint separately, however, when buying an unusual people are mainly looking for the effect & price and completely ignore whether the hat is painted or not (-> paint doesn't matter in unusual suggestions).