That is because the power delivery is different.
With dyno's it is the rate that you can turn the tyres, if you have a close ratio box and diff with short tyres then you are able to spin the engine up faster.
The tyres with my dyno run were actually taller then the standard 14" tyres so I lost a fraction of power.
A dyno isn't exactly the best at showing power though as it is a tuning instrument, there are so many different factors in them that you are better off doing a few runs at the drags and using 1/4 mile converter to get the HP.
The dyno that I ran my car on was also supposed to of had read low, I am not too fussed on this as I will take 90hp as a good figure for a FWD auto that has 300'xxxk's.