VBR is for muxed channels so that it can pull bits from the other channels that don't need them and move bits back & forth to which stream needs it the most. So the VBR is constantly moving the bits back and forth between the streams.
The strange thing is that the place I used to see this most often was on the old 32362 SR 4.2.0 CBS HD transponders that only had one video channel. I used to delay football games (can't do that now that they are 4.2.2) about 25 seconds to sync with alternative audio, and I could barely keep up with storing the video into a computer buffer while the programming was running, but during commercial breaks or something, they would switch to a static slate and the bitrate would drop from close to 40 mbps down to just 2 or 3 mbps if I remember right, and I'd see my buffer go from almost full down to nearly empty.
I can't figure out why they'd use that on a mux with only one channel. All the un-needed bits went into the NULL stream. Now that they have 2 channels on the muxes, it might make sense, but not with the 4.2.0 transponders with one channel.
Actually, it may be related to the same question: whether a broadcaster pays for bandwidth actually used, or just for rented bandwidth "capacity" regardless of use, unless went over the imposed monthly limit?