Wow, thats crazy! I never noticed that before.
I just tried it on my BB Tour and went from about 95 to 82. So good news is, its not just you Inc. owners.
Its amazing how much it changes though.
Holding like OP suggested only with thumb and index finger on bottom and top is was about -82
Holding in typical BB prayer method as if typing on keyboard it went to about -92
Hold in one hand with hand flat as if hand was a table and phone resting on it, it went to about -95
Holding in one hand with fingers slightly wrapped around sides of phone as if you start gesturing on screen (I wish) it went to -100
This is obviously not scientific, but I did it several times in each position and got similar results each time.