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G_M_C,
I assume your comment relates to my explanation.
Indeed, top and bottom margins we can encode as a separate slice. After at most 64 MBs CABAC should tune itself to the actual entropy of the black area (since context models are initially initiated according to cabac_init_idc and SliceQP and are far from the actual entropy). In such case extra bits are generated only for first 64 MBs.
As for right and left margins 'slicing' is not a good idea.
Generally speaking everyone can conduct the following experiment:
1) Take two "different" video excerpts say with the same resolution 480x720 and the same number of pictures. Here "different" means with different entropy (e.g. one stream can be from cartoon another from live video).
2) Encode these two video streams in CABAC mode with constant QP (to exclude RateControl impact).
Suppose that the first stream gives X bits while another Y bits.
3) Then compose a new video by concatenating raw data of the first excerpt and the second one. The resulting video is in the resolution of 480x1440 (where left part is from the first stream and the right is from the second one).
4) Encode the concatenated video excerpt in CABAC mode with the same QP.
It is expected that the new stream gives much more bits than (X +Y), because entropy is changed across each row and CABAC can't stabilize.
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