No - the point is that true constant quality has (almost) no constraints on local bitrate, so one section of a movie might be twenty times the bitrate of anothe

No - the point is that true constant quality has (almost) no constraints on local bitrate, so one section of a movie might be twenty times the bitrate of another section. Online streaming services continually estimate the current available bandwidth, then choose from a selection of pre-encoded streams to download. For this to work well, the selection must know the maximum local bitrate of each stream to match the estimation. If this local maximum isn't known or constrained, you get buffering because you're attempting to download a stream that's actually currently twenty times more than the available bandwidth. Whereas your personal video collection probably isn't being streamed over any link slower than several hundred MBit/s, which is more than enough for anything short of intermediate codec bitrates, plus significant buffering doesn't count against any data caps.