Type of dyno and the conditions in which the measurement was taken, ie: heat, humidity, etc.
No one dyno will read exactly the same from run to run, most are pretty good however and you should use the same dyno each time. Dynos are great for relative comparisons, ie: Add an exhaust and see what the difference is, etc. A drag strip will do a similar thing however the variables are too great, rider, track and weather conditions, etc. The dyno will also allow you to see other areas of engine performance including, torque, mid range performance, shape of the power curve, etc.
Dynos are just one tool in tuning your engine. Other tools include O2 sensors, 4 Gas EGA, plug checks, etc.
The key is to find a shop/professional who can help you make good choices to get what you want out of a tune.
