Hi,
I see you are using SOX to see file's attributes and saw the same issue as well. AME adds the extra Adobe-specific metadata (called XMP) into files. It seems this extra data (bigger file size due to the unknown extra data) is confusing SOX.
Could you try the following steps to disable XMP? This should give you a proper bit rate in SOX.
1. Import a file into AME.
2. Open the Export Settings dialog.
3. Select MP3.
4. Click "Metadata..." button.
5. Select None for "Export Options". This disables XMP.
6. Encode.
It seems this is an issue on SOX. The actual bitrate in a file is correct for me. Other tools (iMediaHUD, MediaInfo, VLC, and etc) gave me a correct bitrate value even with XMP metadata enabled.