I'm not sure why, but it may have something to do with the design goals of the different subs.

The UFW-10 is designed for linear output from 30-35hz and up. Since it is sealed, it rolls off much more gradually, and very linearly, below it's rated low freq. point. The 500 watts and high excursion woofer doesn't hurt either. I don't really know for sure, especially since the STF-2 is a ported, downfiring design with less power exhibiting the same thing!