I need help about image steganography

I read publication in terms " A Noval Spatial Domain Scheme for Multiple Bitplane Steganography" ,and I have some ambiguity about :
I know when we embed 3 bits in 3 lsb bits from each pixel it results in that pixel value either increase or decrease to values(1,2,3,4,5,6,7),so I understand why they put these red circules in attachment picture but I did't understand how Do you calculate propability and why number of probabilities are 32 so that you divided by 32 ????