I think I got it! All genuine items - traded or non-traded have the origin "Steam Purchased". After one update (I dunno which one it was) the origin changes. So if you bought one game + genuine item and you didn't trade it yet the origin should change to "Traded". This doesn't happen with items which are gotten before the update and traded more than 1x.
So old genuine items, which were traded have "Steam Purchased" and will never change!
And items, which aren't traded yet have "Steam Purchased" but after one trade they'll get "Traded" as origin.