Maybe a little on the low side but thats about average. The thing you have to realize is that when the vehicle is moving at highway speeds that 137 pounds of cargo will cause huge amounts of downforce which the rails have to absorb without ripping off the car or breaking. I'm sure the static load the rails could handle in your driveway could easily double the 137.
I've been told they don't allow a lot of weight on the roof mostly for roll-over reasons. It deffinantly effects your center of gravity. I know this has true to it but not sure how much of a factor it is.