Help D2G Signal Bars

[CowboyCombat]

Ok so I have a question regarding the Signal Strength Bars on my Droid 2 Global (or as I've been told with many other phones as well). I am very new to smartphones so I tend to not know anything about them yet. I've noticed over the last few weeks that the Signal Bar reading on my phone rapidly fluctuates between 0-4 bars while it's just sitting on my desk or in my hand or even outside. Yet my dBm reading stays at constant reading depending on where I'm at.

85-90 at home yet 4 bars
65-75 outside yet 0 bars

Now I understand that those are pretty typical readings in terms of dBm, I just dont get why my bars fluctuate so rapidly. Is it software related? Is my phone defective? Is it common with the Droid 2 Global? What can I do to fix it? Any help is greatly appreciated.