Don't lecture an economist about supply and demand, or I'll have to explain arbitrage to you.
Players who were worth $300K were signed, the few remaining players are late birthdays, or thought to be committed (like the guy the Phlllies signed, committed to Tulane, a signal that it would be expensive to buy him out of that commitment, no different from the draft).
Why would you wait to sign someone so you could spend double, and if that player knew he could get more by waiting, so do other players and there would be a greater supply later in the period, negating the advantage of waiting.
Early in the period you don't know who you'll be able to sign, you can make some guesses, but you're going to make errors, later in the period, you know who you haven't signed, and you know who you're potentially competitive for (it's not a free market in LA and even Asia, information is hard to come by and teams depend on their networks of contacts). So at that point you re-evaluate the value of your allotments.