Remember that the bars don't necessarily reflect the actual data service--they indicate what your cell phone reception is like. Currently, voice connection is typically 2G and your broadband data is 3G or 4G (although my understanding is that Verizon plans to roll out voice over 4G LTE in 2014). The bars indicate the voice signal (2G), so you could easily be in an area where you have decent 2G reception, but spotty 4G reception. If 4G is spotty, it will revert to 3G.
You can get a better idea of the signal breakdown by going to System Settings/Wireless & Networks/Mobile Networks/Network Type & Strength. Right now on my Razr Maxx, it tells me 1x (i.e., 2G) is -86 dBm which is 3 bars, and 4G is -115 dBm, which is 2 bars.