That doesn't make intuitive sense to me - I don't think humans have such a zone for (all) binary choices. An example that comes to my mind is the rotating Spinning Dancer Illusion[1] - I am 100% confident that she's spinning to the left, or 100% confident she's spinning to the right - there is no middle-ground where my brain tells me "it could be either direction".

Multistable/Bistable perception is not unique to Google Cloud vision - it afflicts humans too.

1. https://en.wikipedia.org/wiki/Spinning_Dancer