CBR exporting at a set rate does not result in the correct bitrate on the final output file. This appears to be a known bug.
When choosing CBR and selecting a desired bitrate for H264 output, the reesulting file video settings do not match the output, i.e., a 25Mbs CBR setting results in a 10.5Mbs VBR file when inspected with Quicktime Player info, or MediaInfo.
Antoine (Autokroma.com) commented
Would you have file samples ?