Seems that Bitcoinwisdom is using incorrect way of calculating next difficulty, so their predictions immediately after difficulty change are totally off. The reason probably is they take last X blocks speed generation to estimate next difficulty but they are not recalculating blocks generation speed with current difficulty. As the number of generated blocks in current difficulty rises to X, their prediction becomes correct.
You can make an idea about next difficulty change just by looking at bitcoin.sipa.be charts like this one
http://bitcoin.sipa.be/speed-lin-2k.png ...Currently it seems for very little difficulty change because current hashrate is fluctuating near the red - current difficulty line.