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

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.

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

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.