Even though we like to think of RF signal as "line of sight" and always traveling in a straight line, the reality is that the signal will bounce, refract, and even bend as it passes through and between free space (air) and objects on the way to your receiver.
To understand why you're seeing differences in signal strength while you are moving (even slightly) - you need to understand multi-path and nulls. Basically, you are hitting spots where the signal is partially cancelling itself because two (or more) wavefronts are reaching the same point (your receiver's antenna) and they are out of phase with each other. When they're exactly 180 degrees out of phase, they'll cancel each other completely.
The opposite is also true. When two wavefronts reach the receiver at the same time in phase, the net effect can actually be an increase in signal strength. That's part of the reason you'll find "sweet spots" where the radio works best in that exact location, even though you're miles from the transmitter.
If you ever suffer from insomnia, feel free to Google up Multipath and Diversity antennas and phase shifted noise cancellation. There's some interesting stuff that goes on in those areas, especially when it comes to data and higher (mm) frequencies.
Radioguy7268's post in Reception while driving was marked as the answer