Basically.. there is probably just a software glitch that the bars dont truly represent your signal strength. If you look at the actualy dB signal (how to get there is discussed in the link I posted), you will notice that the Incredible can have a great signal and might only show 1-2 bars. The 'larger' the dB signal the better your signal is (e.g. -60 is better than -90)
I noticed that in the same spot, using SpeedTest, my Incredible download speeds (with two bars) was almost twice as fast as my wifes Eris. Her signal strength read -89 and mine read -77.