Obviously, they've learned from past releases of clue-games on JKRowling.com and the SecretStreeView site just how quickly the answer to whatever they post will spread like wildfire throughout fandom. They know that we're going to sit here guessing at and entering any possible answer combination until we hit on the right fit.
Pottermore has a great challenge in order to try to make this fair for everybody. As we had been discussing on this site for weeks, the early entry clues had to be done in such a way that it's not over as soon as it starts, like what happened with SecretStreetView. Fans shared those letters as soon as they found them, figured out how to find the rest, some before they were even released, and turned what was supposed to be a 2-day challenge into a 2-hour frenzy!
We figured that the quill quest challenge must involve some sort of skill on the fans' part and would have to be fair to people across the world with various translations and editions of the text. As pointed out by Brian in the comment trail, that is why the numbers work so beautifully. Words within the text will not be the same in foreign language editions. However, numbers are universal (as long as the translations have remained true to the original and listed the exact same number of owl breeds!
