+1 to benjiboy and Mustafa. In perfect tradition the question arose how to rate a specific film and the range of responses covered a whole range of over- and underexposures.
To all those who rate at ISO 101.5, 89.765 and 105.9: Do you use calibrated light meters, fine tuned E6 processing with fresh chems and can you tell to which exact density you develop the region you base your light measurement off? You use fresh film and certainly develop your film (there was a url link here which no longer exists), don't you?
To all those who don't do the above things: see what Mustafa and benjiboy suggested, and change ISO rating only if you can achieve moderate consistency and still encounter problems.