Chromosome length in Genetic Algorithms

In order to find the appropriate length of chromosomes in GA programming, the author of this book states:

Suppose six decimal places for the variables' values is desirable. It is clear that to achieve such precision each domain Di = [ai,bi] should be cut into (bi - ai) * 10^6 equal size ranges. Let us denote by mi the smallest integer such that (bi - ai) * 10^6 <= 2^mi - 1. Then, a representation having each variable xi coded as a binary string of length mi clearly satisfies the precision requirement. Additionally, the following formula interprets each such string:

xi = ai + decimal(1001...001) * (bi - ai)/(2^mi - 1)

where decimal(string) represents the decimal value of that binary string.

So here is my question: Why is the author using (bi - ai)/(2^mi - 1)? Why not (bi - ai)/(2^mi)? What is that -1 for?

I searched it and I thought it might have something to do with the Mersenne Prime numbers because of the formulation!! I also checked out the Schema as I thought it might be related to that, but these all seem completely unrelated!