According to
http://bitcoin.sipa.be/, it seems some big miners will purposely slow down the hashing just before the difficulty adjusting to reduce the jump? Although I know that does not make much sense, since what they get is based on their hash rate percentage, not the difficulty. Maybe they are just doing that to save the environment.
That doesn't make any sense, the difficulty adjustment is based on the previous 2016 blocks and their average rate of discovery, it's not based on whatever the hashrate is right before difficulty jumps.