The prolog block has a header and a footer and an empty payload and so the size of the prolog block is 8.

The prolog block is marked as allocated.

So in binary, the header is

        0000 0000 0000 0000 0000 0000 0000 1001
     

As an integer, this looks like 9, but the lower order 3 bits are not part of the size. The size has 3 implied (not stored) low order bits equal to 0:

        0000 0000 0000 0000 0000 0000 0000 1000
     

which is the value 8

It is convenient to write the header value as 8/1: size 8 and allocated bit = 1.