Because RIG is already down a lot on a percentage basis. I also believe NOV could be more widely held.
Notice the LEH fund today that had to shut down because it had lost money in Copper and Natural Gas.
How in the hell do you lose money in Copper and Nat Gas with their market exploding?
Anyway that was a 4 billion fund having to sell. Do you think they owned any FCX or RIG or NOV. My guess is yes and there are several "funds" getting their asses kicked that are having to sell. This of course is driving the prices down. You can be sure there are others that sat back today and are on pins and needles ready to pounce on NOV. It will come. And when all the have to sellers leave this baby will rocket higher. No matter what oil is a barrel.
I don't think it has any thing to do with fundamentals of the company. Nov was up and goes up a lot more than rig on good days and down moer on bad days. People taking profits and, perhaps, smaller float.