Dyno rooms are not only filled with the sound of screaming engines, but they're also rammed to the rafters with bullshit. Every dyno gives a different reading and there are a series of 'correction' factors that can be applied to alter the results.
These are EC standard, DIN and SAE, which all give slightly different results and are there because temperature and humidity alter dyno results.
These corrections are applied to the result in an attempt to standardize dynos all over the world and take into account the climatic conditions. The problem is that they give slightly different end figures. If you're getting your motorcycle dynoed, make sure the figures before and after the test uses the same correction factors so you know where you stand.
Then there is where the power is quoted. By quoting an 'at the crank' firgure, the dyno man can add around 12% to your motorcycle's power, about the amount of power lost between the crank and rear wheel within the engine. Most people use 'at the rear wheel' power figures. Ever wondered why your motorcycle never makes the same power the brochure claims? Most magazines quote rear wheel figures, while manufacturers quote crank figures because they sound better. This is why an Yamaha YZF-R1 is claimed to make 182bhp but only really makes 165bhp.