Consider hex and base 64. Base 16 takes the first six letters from the alphabet to augment the number of available letters for representing a particular value.

I guess using base 26 would make sense from a strictly alphabetic standpoint, but we've conventionally included the numbers 0-9 as part of the available letters for representation.

by convention in base 36, numbers can be represented by [0-9] and [a-z]. 1(36)=1(10) a(36)=10(10) and z(36)=36(10).

I guess they think words somehow include numbers