Repost from Chris Bedford at Encoding video and audio content with Adobe Media Encoder:

There is a known bug when exporting from Premiere Pro or After Effects involving modified bitrate values. If you modify the bitrate value of an encoding preset, the default bitrate for that preset will replace the custom bitrate you specified.

As a workaround, you can:

Manually change the bit rate of the video file you are exporting to the desired value once the file is in the Adobe Media Encoder encoding queue…

-or-

Add your video to the list of files to encode from Adobe Media Encoder itself. You can drag a file into the list, or click the Add button and select a file on your computer.

Note: To add Adobe Premiere Pro sequences or Adobe After Effects compositions to the list of files to encode, you must use the File > Add Adobe Premiere Pro sequences or File > Add Adobe After Effects Compositions menu commands. These file types cannot be dragged into the list of files to encode.

For more information see, Workflow for exporting files in the Adobe Premiere Pro help.

Personally, I’ve been having a problem exporting from Premiere Pro through the Adobe Media Encoder, period! It starts in the Media Encoder, then fails. As a workaround, I’ve had to use dynamic link and send it to After Effects, then go through and fix any transitions and titles I may have used. I still love to edit in PPRO, but I hope this issue is fixed asap. Anyone out there have a better solution for me? Thanks in advance.