VBR encoding produces an overall higher and more consistent quality level than Constant Bit Rate encoding. As a result, your MP3 file consists of variable bit rates throughout the file because RealJukebox allocates the bits where they are most needed. Variable Bit Rate encoding was developed for use when consistent audio quality is the top priority and constant or predictable encoded MP3 file size is not critical. Many MP3 players may inaccurately report the duration of VBR-encoded files. If the MP3 player cannot accurately determine the time, seeking in the file may also be unpredictable.
Some audio passages are more difficult to encode than others.
By maintaining a constant bit rate, you force the quality of the CBR operation to vary according to the complexity of the audio file. Difficult passages encoded using VBR allows a higher than average bit rate and during easy passages VBR allows a lower than average bit rate. The net result is that VBR produces an overall higher, more consistent quality level compared to CBR at similar bit rates. If you set you file to encode with VBR the bit rate will vary and the quality won't. I have encoded music with Variable Bit Rate.