Ah, seems if you switch to JPG it "unlocks" the slider so you can change it to 100%. Then when you switch it back to PDF it "locks" the quality slider again. That doesn't sound like a feature, more like a bug, no?
While on this topic. Can you shed some light on how to change the Export quality from the default of 85% to 100% for PDF? Seems to be locked. Not sure why any users would want to make a Skitch screen grab and then export it with a 15% loss in quality (ie introducing unnecessary blur) for no apparent gain (a few KBs in size?)