Did you also select the "Restrict encoder to constant bitrate (CBR Encoding)" option?
If it is not ticked MJ uses the average bitrate mode (ABR). It is a restricted variable bit rate mode that tries to reach the selected average bitrate. However, if the audio content happens to be easy to compress the resulting average bitrate may be slightly smaller than the selected value.
The general consensus is that the unrestricted variable bitrate aka VBR mode produces the best quality/file size ratio. You select the preferred quality level and the encoder can use any momentary bitrate when it tries to keep the quality constant. The VBR mode does not have a specific target bitrate.
For instance, usually VBR "High" produces average bitrates that are in the 180-210 kbps range, but this can vary greatly depending on the audio content's complexity.