Variable Bit Rate - VBR is best for progressive download where you have some flexibility over the data rate at any given moment. Variable Bit Rate means the encoder can allow for some increase and decrease in data rate as needed and dictated by the settings you chose.
With Windows Media, 1 Pass VBR, the Quality Slider is the target. The encoder targets the quality setting and does whatever it needs to the bandwidth to get there. You could get very different data rates with two different videos if one were talking heads and the other was fast action sports. The fast action would need a higher data rate to get the same quality setting.