Dynojet is Inertial Dyno, Dyno Dynamics is Load Dyno (or so I've heard)
The way they calculate torque is different and due to external factors the hp numbers will differ slightly.
One question of 'accuracy'.
Is 'true' HP output can be seen without evaluating the car's drag co-efficiency? (a.k.a. wind drag)
Dynometers are giant torque measuring equipment.. they measure torque. And due to the different methods that they use to obtain the numbers, there's always slight deviations due to air pressure, temperature, which all relates to air-density.
In fact, your car's ECU changes the fuelling based on it's many sensors, so that actually compounds the slight inaccuracies you have when doing multiple runs on the dyno.
Your first run, and your tenth run will yield differing results as the engine bay heats up, and changes your intake temperature, in turn changing your fuelling.