I'm fairly sure that Band 6 will always be 90+.
They get around it by doing all sorts of "mapping" (which is different to scaling) from the raw marks. They decide what sorts of answers mark the boundaries between various bands according to the Performance Descriptors, and then "map" the results from each student onto the various bands. Does that make sense?
Your right, ElendilPeredhil ... theoretically, every student could get Band 6 if they all answer the questions according to the standard required, but with 30000 students doing this course, that ain't going to happen!
I'd be fairly sure that the Board will set it up in such a way as the same statistics occur ... about 2-4% of the candidature will get Band 6.