Why the overlap set of SDR isn't w choose b?

Science Numenta Theory
1

i’m learning SDR overlap set at HTM School.
give a SDR “x”, x is one of all encodings, so this SDRx is unique and fixed, right?
so, the number of all SDRs with b bits overlaps with SDRx is w choose b, am i right?

w is population, b is overlapped bits number.

2

i was wrong , the next step is to choose the off bits.

3

If the bits code or some feature of the perceived universe then bit overlap strongly suggest that this perception shares features.

This supposition depends on the granularity of the encoding.