Read the link that Dutch posted. The lower rating is a safety factor to take into account the slight differences in tire loads between duals from slight differences in diameters, between tires or crowning of road services. It has nothing to do with air gap stated in another post.
I have the pull-rite slider and firestone bags on my truck. There is no interference at all. I also use the baqgs just to keep it level and I am under the axle weight ratings. It helps especially on bumps keeping the rubber bump stops from hitting and making the ride smoother.