I am generally sceptical of stats by biologist anymore. I find more often than not that the study was intented to verify a predetermined outcome. Some of these "biologists" dont belive we should be allowed to carry guns for defense and may want to make it appear guns are ineffective.
That being said I would imagine that the bears senses (smell, taste, sight) are much more senstive than ours and that may make the sprays very effective on bears despite their size.