In comparison,
if you have 12 kills, you most likely got around 3600 gold, depending who you killed and what stats they had.
The Katarina getting 1700 gold from killing you twice only gives her very little in comparison to what gold you have.
At this point you could argue that you somehow threw the game as a 0 kills Katarina shouldn't be able to kill you when you have 12 kills.
The most likely scenarios are that:
you were out of position and got snacked by the enemy team
you overestimated your mechanics and lost to the Katarina, because you're just not good enough
the Katarina had way more CS than you, thus getting much more gold from that particular source, which enabled her comeback
or others
The thing is people wanted something to enable comebacks again. Before the bounty system, it was much harder to make a comeback when you were behind. Now even when you pick up the person with the bounty, it's still hard to make a comeback, because when there is a 12/0 enemy carry, then it's really not easy, unless they throw.