In the assembly click on just the skid part and make sure that your not getting the same mass as the part.
#2 I updated my start part mass to be that of steel so that I would never have to worry about adding material and the weights that I tested back in that day where what I considered acceptable when compared to actual parts and assemblies. The problem with some parts that I have found is that they are hard to model correctly and accurately for mass. That is why I kind of like the override mass properties. make sure that any other parts are set correctly and if your going to add material to one configuration add it to all of them.