Don't quite know about Campy 7-speeds, but the later lot tends to have more dish than their Shimano counterpart. This causes them to build up with greater DS/NDS imbalance which is bad for NDS spoke durability(spocalc will tell you how much). So in the interest of durability it's sensible to use different gauges on DS/NDS, with DS getting a thicker gauge. The theoretical optimum is when NDS spoke cross section is is the same fraction of DS cross section as NDS spoke tension is of DS spoke tension.