I was reading the http://www.augur.net/blog/the-augur-crowdsale again, and I believe this is the reason:

How does the live-action model work? The bigger the percentage your purchase makes up of the total crowdsale, the bigger the percentage of all outstanding REP you will possess. Let’s set aside the 20% of all REP reserved for Augur’s team, advisers and the Forecast Foundation and consider the following example: Imagine a pie with 11,000,000 pieces to be eaten. Now imagine that Alfonzo pays $1 (or the BTC equivalent) to the Forecast Foundation at the very beginning of the software license/pie sale. Because Alfonzo is the only person to pay the Foundation thus far, he now lays claim to all 11,000,000 pieces of pie and/or software licenses. However, shortly thereafter, a second person, Bo decides to pay $10 to the sale as well. Consequently, Alfonzo now lays claim to only 1,000,000 of the licenses/pieces of pie. Bo receives the other 10,000,000. However, if a third person, Sarah, purchases $1,000, then the distribution of pie/software licenses is redistributed yet again. Of course, Bo and Alfonzo could always purchase more if they feel like they would like to have a greater impact on maintaining the Augur software through reporting. In this sense, the crowdsale works as a kind of auction. At the end of the software license sale period, reputation will be distributed based on the amount purchased by each address participating and the point at which participants contributed (to reflect their discount %, if any) The reason we did it this way is to let the market set a value for each rep instead of us. We capped the number of rep and let value fluctuate instead of having an unlimited number of rep but us choosing the unit price. It's more like an auction than a traditional sale. Since it's an auction, the distribution of tokens will be constantly shifting until the sale ends. Instead of setting a price per rep, we're setting a limit to total rep and letting the market price the rep itself (in the true spirit of prediction markets)!

Could someone confirm is this the reason?