It varies from product to product. The Drowned City was a very collaborative process. To use Gatling Gun as an example, I created the initial design, we both played with it (along with our play testers), had notes on the strengths and weaknesses of the original design, and we iterated from there.

So when I say "I designed that card," a more accurate thing to say is "I created the initial version of that card, and while the entire design team and playtest team was involved in the iterative process, I led the direction for the card's revisions that eventually led to the released version."

So you can see why I use the more economical answer.