Taken from the Skunk2 website, some guy on a Civic forum accused them of ripping people off by selling 2.5" exhausts that were rounded up to 2.5, they disliked his allegations 

It all started with an 8thcivic.com member and Skunk2 ’06+Civic Si header owner who “digitally measured” his header’s collector to a 2.395-inch ID and 2.435-inch OD. Ten pages later and it looked like the facts were in. Indeed, the evil and deceitful Skunk2 is guilty of “false advertising” and “rounds up” at the expense of innocent Civic Si owners. The general consensus was that Skunk2 also hates babies and puppies, pushes old people around, and has its headers made in sweatshops by children. Of course, do the math and for any of this to be true this guy’s K header would have to be made out of paper-thin metal—.020-inches, for all of you curious internet heroes. We’re doing cool things over here, but making headers so thin you could almost see through them isn’t one of them. Ultimately we had to head to the forum ourselves and school the e-tards with some basic math.
It’s unfortunate when “experts” jump online and accuse companies of false advertising and, before both sides are heard, the haters chime in with their bashing based on unconfirmed information. The truth’s been revealed but we’ve yet to hear back from the haters. And we don't expect to.