Why does Renegade Freighter not just have trample?
There are some more relevant edge cases where the difference actually matters. Two of the cards that were in standard at the same time as Renegade Freighter were Odric, Lunarch Marshal and Majestic Myriarch. Both of those check for abilities including Trample at the beginning of combat, so in both cases, they can find Trample on Untethered Express, but not on Renegade Freighter.