[x265] Under VBV rc mode , increasing the vbv-maxrate or vbv-bufsize has no or very little effect on the final result

[fangzhen]

Recently, I am testing the x265 codes  and mostly on VBV rc mode.\
But, I find an strange problem. For instance, I set up the following test circumstance:
resolution: 1920x1080
bitrate=1000kbps, vbv-bufsize=3000kbit
when I set vbv-maxbitrate>1250kbps, I get almost the same result, like the same psnr, the almost same bitrate.
I get the same result for vbv-bufsize where the vbv-maxbitrate is fixed, like 3000kbps.
So I wonder why this situation exists? Can anybody give me some explanation? Thanks so much in advance.
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