It may not necessarily be inefficient. YC has an established brand. Initial YC investors know they will get the best possible terms, so they would be wise to offer the max value. The start-up it could take this "max" valuation to all future investors.
The alternative would be a low initial offer which would essentially keep anyone else from paying more. In essence the YC start-ups are giving the initial investor a money back guarantee on the difference between their offer and the lowest offer.