That's what I see, too. Even in the smaller light truck/ standard load sizes there's around a three pound difference. There's got to be more rubber in there somewhere.
What I found on one website (Toyo Tires) that I thought was interesting was that in the same size tire, an LT will take more air pressure to carry the same weight as a SL, so the ride will be harsher. But like you said, the LT has the potential to carry more weight than the SL.
