I think it should be fixed.

0 is the first number of dec, and a is first letter in alpha

dec is base 10, so 27 is 27 = 2 * 10 + 7.
allow has 26 chars, is base 26. a is 0, b is 1, c is 2 ... and z is 25. so bb = b * 26 + b = 1 * 26 + 1.

Do you understand better now?