I've also heard the 15-20% numbers. The thing to remember is that not all dyno operators know what they're doing. You might want to check out another dyno location if you really feel the numbers are way off.

I'm a big believer in computer dyno sims, and if entered accurately enter, the information (horsepower and torque levels) can be spot on. I have found this to be the case with my Suzuki race engines (within +- 3 HP).