I would imagine because zebras are based on luminance not RGB gamut. I don't know how the histograms are implemented in BMD cameras and whether they are based on the current lut or absolute sensor saturation, since I don't use them or false colours, I'm too old fashioned and an old ENG guy
But I would not recommend, personally 100% zebras - it is much better to use 90%, so you have some head room, to allow for inaccuracy of metering. It would be grand too if the Broadcast particularly had dual zebras but it is quite workable as is.