Why NvEncodeAPI_v3.0 can not adjust the bitrate of 4K video stream?

I found in NvEncodeAPI_v3.0,whatever the value I set to “averageBitRate” & “maxBitrate”,the output bitstream always in a same bitrate,for example,for example,below is my config to encoder(the trace info in DebugView):

NV_ENCAPI_TRACE: CNvVideoEncoderH264::Initialize: encodeGUID: 6BC82762-4E63-4CA4-501E85AA
NV_ENCAPI_TRACE: CNvVideoEncoderH264::Initialize: profileGUID: E7CBC309-4F7A-4B89-37D52AAF
NV_ENCAPI_TRACE: CNvVideoEncoderH264::Initialize: presetGUID: 82E3E450-BDBB-4E40-A9829C98
NV_ENCAPI_TRACE: CNvVideoEncoderH264::Initialize:
EncodeWidth 3840
EncodeHeight 2160
PTD enabled 1
Async mode enabled 0
level 0
FieldEncoding 0
GopLength 30
No. of Bframes 2
entropy coding mode 1
RCMode 1
Avg bitrate 20000000
Peak Bitrate 30000000

I use the “NV_ENC_PRESET_BD_GUID” preset config,when I set avgbitrate as 20M,and maxbitrate as 30M,the output bitstream only at about 6M,when I set avgbitrate & maxbitrate as zero,or only use preset config,the output bitstream is about 22M; Besides,there is no other bitrate I can get;I known when encoding HD video,I can free adjust the output bitrate,but what happened to 4K video?How to adjust bitrate when encoding 4K video stream?

Below is the HW & SW environment of my test machine :
Geforce GTX700 TITAN
Windows7 - 64bit
Geforce V320.79 Driver
NvEncodeAPI : v3.0 beta,the license key I used in NvEncSDK is “NV_CLIENT_KEY_TEST”,I grabber it from v2.0_1219 SDK;