No one else gets the same results that you do. (that's not fair, there are a few derogatory comments floating around the net- but there are for all these companies)
There is extensive 3rd party chrono testing of DT ammo posted on Glock Talk and other forums.
One totally random example:
"Everything I've shot from Double Tap has performed great (10mm and .45ACP). When the site says that the 135gr. 10mm loads fly at 1600fps they are pretty much spot on. I chrono'd the same loads through a converted Glock 21 and a five shot test group averaged 1580 fps., so the DT numbers are not just marketing hype."
http://www.thehighroad.org/showthread.php?t=327658
Buffalo Bore and Cor Bon all claim similar velocities, but often use inferior bullet designs. Buffalo Bore costs a LOT more money too.