I think I might have a little info here. If the hype is to be believed fluoro is denser than mono which means that it will sink, and the refraction properties of fluoro are closer to water than mono, which makes it mostly invisible to fish. I have looked at the large spools of fluoro in the spinning section of the sporting goods stores, and the lb test is consistently lighter per line diameter than the fly shop spools. This may help explain why the fly shop spools are more expensive. Just my $.02.