Setting of "DM8148 H.264-ENC Bitrate CBR"

I'm running the H.264 encoding on the following platforms.

I have set the 'bitrate' and 'cbr'.
The result was as follows.

[SETTING]   [RESULT]
 100Kbps → 144Kbps
 50Kbps  → 121Kbps
 20Kbps  → 117Kbps

I have thought that CBR setting is disabled.
Setting is performed as follows.

20Kbpsの場合)
 | vencDynaParams.rcAlg = VENC_RATE_CTRL_CBR;
 | err = Venc_setDynamicParam( 0, 0, &vencDynaParams, VENC_RCALG);
 |
 | vencDynaParams.targetBitRate = 20 * 1000;
 | err = Venc_setDynamicParam( 0, 0, &vencDynaParams, VENC_BITRATE );

I want to know what needs to be set to other.