1 / 1 = 1 = b - 1 1 % 1 = 0 = b - 2
21 (base 3) = 7 12 (base 3) = 5 7 / 5 = 1 = b - 2 7 % 5 = 2 = b - 1
In base 2 (and only base 2), denom(b) >= b-1, so the "fractional part" (b-1)/denom(b) carries into the 1's (units) place, which then carries into the 2's (b's) place, flipping both bits.
In base 2 (and only base 2), denom(b) >= b-1, so the "fractional part" (b-1)/denom(b) carries into the 1's (units) place, which then carries into the 2's (b's) place, flipping both bits.