H.264 hardware encoding inaccurate bitrate

Hello folks,

Just upgraded to AME / Premiere Pro 14.2 / After Effects 17.1 I'm encoding an MXF file encoded with JPEG2000 codec to H.264. Since I have a NVidia Quadro K4000 video card, I meet the requirements to Hardware encode. The encoding does work, however I noticed the bitrate setting was completely wrong. I set to 15mbps and the file comes out 12.5mbps. When I set to the highest setting of 50mpbs, the end file is only 14.9mbps. I'm meeting a deliverable specification of 15-16mbps and would love to use the hardware encoding.

Is there a data rate limit or range the hardware encoding is locked?