I emailed D&D to inquire about the dyno numbers. According to their engineer, their numbers are accurate and attributes the differences as follows:
Triumph uses DIN correction factors which is a European standard and differs from the SAE corrections used in the US. I have seen Dyno-Jet numbers in the mid 80's for the same motorcycle. The thing to remember is that most dynos read a little different (This is because of dyno locations, drum weight differences, type of eddy brake being used, type of fuel in the bike, etc.
Also of interest, D&D retains the catalytic converters in their pipes. It was my understanding that the Triumph Accessory mufflers deleted the cats. Curious that D&D states that their pipes require the Triumph Accessory muffler tune and also recommend a K&N air filter. I guess you have to assume they feel they achieved a comparable increase in flow with their mufflers equal to Triumph's removal of the cats in their Accessory mufflers.