Well, ABR can never be 100% accurate, because it doesn't know "what is in the future", it can only make estimates from the frames seen "in the past".
Furthermore, accuracy will usually be much better with longer clips. You have probably been testing with rather short test sequences. If so, those clips simply might have been too short for the ABR algorithm to converge.
Finally, there is the "--ratetol" option, which controls how much the ABR algorithm is allowed to diverge from the target bitrate. You can set a lower value here, but that might result in quality degradation...
Last edited by LoRd_MuldeR; 9th October 2013 at 17:44.
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