VBV, higher bitrate at the same crf?

I've been playing with x264 with different settings to see the results. One of the test really took my attention. Command line:

Code:

x264 --crf 15 --fps 29.97 --keyint 29 --me tesa --subme 10 --ref 4 --bframes 3 --deblock -1:-1 --psy-rd 0.0:0.0 --trellis 2 --slices 4 --aq-mode 2 --b-adapt 2 --weightp 0 --profile high --level 4.1 --min-keyint 1 --b-pyramid strict --aud --direct auto --partitions all --colorprim "bt709" --transfer "bt709" --colormatrix "bt709" --fullrange "on" --sar 1:1 test.mkv -o 2tesasub10.2x.mkv

With the exact same settings, source, etc. but this time with --vbv-maxrate 40000 --vbv-bufsize 30000 resulted in 425 kbps more. Results without VBV:

Code:

E:\tests>x264 --crf 15 --fps 29.97 --keyint 29 --me tesa --subme 10 --ref 4 --bframes 3 --deblock -1:-1 --psy-rd 0.0:0.0 --trellis 2 --slices 4 --aq-mode 2 --b-adapt 2 --weightp 0 --profile high --level 4.1 --min-keyint 1 --b-pyramid strict --aud --direct auto --partitions all --colorprim "bt709" --transfer "bt709" --colormatrix "bt709" --fullrange "on" --sar 1:1 test.mkv -o 2tesasub10.22.mkv
ffms [info]: 1680x1050p 1:1 @ 30000/1001 fps (cfr)
x264 [info]: using SAR=1/1
x264 [info]: using cpu capabilities: MMX2 SSE2Fast SSSE3 FastShuffle SSE4.2
x264 [info]: profile High, level 4.1
x264 [info]: frame I:117 Avg QP:14.04 size:130464
x264 [info]: frame P:1225 Avg QP:18.13 size: 44200
x264 [info]: frame B:1620 Avg QP:21.11 size: 17882
x264 [info]: consecutive B-frames: 17.1% 11.9% 27.3% 43.7%
x264 [info]: mb I I16..4: 27.9% 43.1% 29.1%
x264 [info]: mb P I16..4: 7.1% 12.5% 4.2% P16..4: 20.6% 11.4% 3.6% 1.2% 0
.5% skip:39.0%
x264 [info]: mb B I16..4: 1.5% 1.8% 0.6% B16..8: 31.8% 3.0% 3.0% direct:
3.1% skip:55.3% L0:39.2% L1:42.4% BI:18.4%
x264 [info]: 8x8 transform intra:49.2% inter:55.5%
x264 [info]: direct mvs spatial:99.8% temporal:0.2%
x264 [info]: coded y,uvDC,uvAC intra: 48.0% 44.1% 20.5% inter: 14.7% 7.2% 0.6%
x264 [info]: i16 v,h,dc,p: 40% 19% 10% 31%
x264 [info]: i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 26% 17% 22% 5% 6% 7% 5% 6% 6%
x264 [info]: i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 26% 18% 18% 6% 7% 8% 6% 6% 6%
x264 [info]: ref P L0: 85.5% 12.8% 1.6%
x264 [info]: ref B L0: 93.0% 7.0%
x264 [info]: ref B L1: 94.8% 5.2%
x264 [info]: kb/s:7963.20

encoded 2962 frames, 7.22 fps, 7963.25 kb/s

Results with VBV:

Code:

E:\tests>x264 --crf 15 --fps 29.97 --keyint 29 --me tesa --subme 10 --ref 4 --bframes 3 --deblock -1:-1 --psy-rd 0.0:0.0 --trellis 2 --slices 4 --aq-mode 2 --b-adapt 2 --weightp 0 --vbv-maxrate 40000 --vbv-bufsize 30000 --profile high --level 4.1 --min-keyint 1 --b-pyramid strict --aud --direct auto --partitions all --colorprim "bt709" --transfer "bt709" --colormatrix "bt709" --fullrange "on" --sar 1:1 test.mkv -o 2tesasub10.23.mkv
ffms [info]: 1680x1050p 1:1 @ 30000/1001 fps (cfr)
x264 [info]: using SAR=1/1
x264 [info]: using cpu capabilities: MMX2 SSE2Fast SSSE3 FastShuffle SSE4.2
x264 [info]: profile High, level 4.1
x264 [info]: frame I:117 Avg QP:13.78 size:133504
x264 [info]: frame P:1225 Avg QP:17.75 size: 46395
x264 [info]: frame B:1620 Avg QP:20.53 size: 19323
x264 [info]: consecutive B-frames: 17.1% 11.9% 27.3% 43.7%
x264 [info]: mb I I16..4: 27.4% 42.3% 30.3%
x264 [info]: mb P I16..4: 6.8% 12.5% 4.5% P16..4: 20.3% 11.7% 3.8% 1.3% 0
.5% skip:38.5%
x264 [info]: mb B I16..4: 1.5% 1.8% 0.7% B16..8: 32.1% 3.2% 3.2% direct:
3.4% skip:54.0% L0:38.9% L1:42.3% BI:18.9%
x264 [info]: 8x8 transform intra:49.0% inter:53.4%
x264 [info]: direct mvs spatial:99.8% temporal:0.2%
x264 [info]: coded y,uvDC,uvAC intra: 49.1% 44.8% 21.6% inter: 15.3% 7.6% 0.8%
x264 [info]: i16 v,h,dc,p: 40% 19% 10% 31%
x264 [info]: i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 26% 17% 22% 5% 6% 7% 5% 6% 6%
x264 [info]: i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 26% 18% 18% 6% 7% 8% 6% 6% 6%
x264 [info]: ref P L0: 85.5% 12.9% 1.6%
x264 [info]: ref B L0: 92.9% 7.1%
x264 [info]: ref B L1: 94.9% 5.1%
x264 [info]: kb/s:8398.69

encoded 2962 frames, 6.90 fps, 8398.75 kb/s

I'd like to know where those 435 kbps come from. Thanks!