How to inclease the bitrate of video of still image using h264_nvenc encode

Hi,
Could you please let us know the way of solving a following problem.
We would like to encode a raw video by specified bitrate.
But when we encode the raw video like still image by a specified bitrate, bitrate of encoded video is lower than our specified bitrate.
Could you please let us know how to get specified bitrate videos(like CBR, ABR).

We tried as follows(NVIDIA QUADRO M2000, NVIDIA-Linux-x86_64-367.27, ffmpeg3.1, CentOS7 1511 x64);
1)
[encode command]
ffmpeg -f rawvideo -vcodec rawvideo -s 1920x1080 -pix_fmt yuv420p -r 59.94 -i a.yuv -c:v h264_nvenc -rc 1 -cbr true -b:v 20000k a.mp4
[ffprobe result]
Duration: 00:00:01.07, start: 0.000000, bitrate: 1248 kb/s

[encode command]
ffmpeg -f rawvideo -vcodec rawvideo -s 1920x1080 -pix_fmt yuv420p -r 59.94 -i a.yuv -c:v h264_nvenc -rc 1 -cbr true -b:v 20000k -minrate 20000k -maxrate 20000k -bufsize 1835k b.mp4

[ffprobe result]
Duration: 00:00:01.07, start: 0.000000, bitrate: 1253 kb/s

[encode command]
ffmpeg -f rawvideo -vcodec rawvideo -s 1920x1080 -pix_fmt yuv420p -r 59.94 -i a.yuv -c:v h264_nvenc -b:v 20M c.mp4

[ffprobe result]
Duration: 00:00:01.07, start: 0.000000, bitrate: 1248 kb/s

[encode command not use nvenc]
ffmpeg -f rawvideo -vcodec rawvideo -s 1920x1080 -pix_fmt yuv420p -r 59.94 -i a.yuv -c:v libx264 -nal-hrd cbr -vsync 1 -b:v 20000k -minrate 20000k -maxrate 20000k -bufsize 1835k d.mp4

[ffprobe result]
Duration: 00:00:01.07, start: 0.000000, bitrate: 19841 kb/s

[Environment]
Server: Dell PowerEdge T630
OS: CentOS 7 1511 x64
GPU board: NVIDIA QUADRO M2000
YUV frame size: 1920x1080
ENCODE FPS:59.94fps
ENCODE GOP SIZE:12 frames(default)
ffmpeg 3.1 (libx264 20160629-2245-stable)
NVIDIA Video Codec SDK 7.0.1
NVIDIA Linux Driver 367.27

NVENC parameters
-preset(default medium)(default,slow,medium,fast,hp,hq,bd,ll,llhq,llhp,lossless,losslesshp)
-profile(default main)(baseline,main,high,high444p)
-level(default auto)(auto, 1, 1.0, , 1b, 1.0b, 1.1, 1.2, 1.3, 2, 2.0, 2.1, 2.2, 3, 3.0, 3.1, 3.2, 4, 4.0, 4.1, 4.2, 5, 5.0, 5.1)
-rc(default -1)(constqp,vbr,cbr,vbr_minqp,ll_2pass_quality,ll_2pass_size,vbr_2pass)
-surfaces(default 32)
-cbr(default false)
-2pass(default auto)
-gpu(default any)(any,list)
-delay(default INT_MAX)

Sincerely regards.