I'm going to hazard an explanation with the understanding that I could be dead wrong. Angie, if I am, please set everyone traight!!
My understanding is that it's more a matter of overall proportions than anything else. Hourglasses have hip and shoulder measurements that are proportional to each other, while pairs have hip measurements that are a little bigger than the rest of the body. I am an extra curvy hourglass, which means that while my waist is the smallest part of my body, my hips and shoulders are still not what you'd call small (in fact they're likely bigger than some trim pairs). As a result, I still have to consider a lot of pair guidelines when dressing myself in hourglass-friendly styles (there's a fair bit of overlap, at least). Someone like Angie, on the other hand, who is smaller all around, can play more with rectangle guidelines because her hips are nowhere near pair territory. In other words, it's not so much about hip measurements on their own as about hip measurements relative to the rest of the body.
Hope that made at least a little bit of sense.
