What happened to the nonce distribution between blocks 24480-69120? See: http://pastebin.com/raw.php?i=Sk6vyBRu (each row represents 1440 blocks, each column represents a range of 100M values, covering blocks 0-216000)

I've been doing some simple nonce analysis, as you can see, and it appears that something happened to nonce distribution between roughly blocks 24480-69120. As you can see nonce distribution between the range 0-2^32 is usually very evenly dispersed, so it appears something happened here and was corrected.

Can anybody point me at anything related to this or inform me of what happened?

For ease, this correlates to the start of October 2009 through till mid July 2010.
The only logical explanations to me are thus:


Some technical limitation meant that the miners at the time (there weren't many) weren't searching some nonces as they should. Everybody was using the built in miner then, so it's probably easy to check if there's an old enough SVN around. 
The miners at the time intentionally binned certain nonces to avoid hoarding all the coins, keeping the difficulty low. The difficulty was barely 1 at that point, so I don't think this is an unreasonable assumption.