People interested in more of the details can read the Nature Methods article here: http://www.nature.com/nmeth/journal/vaop/ncurrent/full/nmeth...
It also fits into a long history of work out of the Mogil lab about the limitations in our current assays for pain. It's worth noting that this is by no means an easy problem. Pain is inherently subjective, and measuring and quantifying it in humans is difficult, let alone in rodents. The only way forward is more work like this - that carefully quantifies the sources of potential bias and error in our assays, and proposes solutions for how to get around them.
Edited to add: Vox has a good interview with Mogil that goes more into this. You can read it here: http://www.vox.com/2014/4/28/5653444/mice-get-stressed-out-f...