It's obviously because the phone was erroneously indicating that you had more signal that you really had. Then when people held the phone in such a way as to cause the signal to drop a little, they were actually dropping the signal down to where it was almost gone even though the bars were still high. Continuing to hold it that way would cause the bars to drop down to where they actually should have been all along.
It makes perfect sense to me and was along the lines of what I figured the problem was going to turn out to be. Sitting at my desk at home, I always had very marginal signal with my other phones. With the iPhone 4, I would often look down and see 4 or 5 bars and thought "wow, this new phone gets much better reception here". The reality is, it was just as weak as before but was being displayed erroneously. Then when I would hold the phone in the bottom left corner, I would see the bars drop down to show the signal accurately.
It also explains perfectly why when you are in a strong signal area, the bars won't drop at all when held at the left corner.
^this.