Lots of miners play the on/off game right now, each time the difficulty drops they come back and switch off again when the price goes down.
The hash rate data doesn't support that hypothesis, IMHO. It's a logical conclusion to draw, but when you look at the hashrate data (see the charts linked above) it doesn't look like any substantial amount of miners has come online since the difficulty drop. We're actually right now (according to BitcoinCharts) still below the 6/hour target.
It looked like that on this graph the moment I posted it.
Onset was before the actual drop, so my hypothesis was/is that they wait till the estimate is sufficiently reliable.