Still it does not make sense. GHS/BTC price should theoretically follow the difficulty increase. So for 25% diff increase the price should have decreased by 25%, i.e. from 0.0075 to somewhere around 0.0052.. Even though it dropped to 0.006 for a minute, it is now around 0.0067 which is less that 10% drop.
This means the market is extremely optimistic about the ROI of their investment..
It is slightly more complicated then that because of NPV. If you still have the option of selling your GHs at any time then the value will be increased even though the stream of revenue has decreased.