One of the publications in terms "A Noval Spatial Domain Scheme for Multiple Bitplane Steganography" contains some ambiguity I can't undrestand , I know when we use 3 bits from byte of pixel ,the value of pixel either increase or decrease to values (1,2,3,4,5,6,7) so the formula of total additive noise to byte is given like as:

I did'nt understand how Do they calculate propability reffered to with red arrow and why number of probabilities are 32 so that you divided by 32 ????