Maybe because it's easier to fit Roman numerals on the tokens, and they have a gravitas that Arabic numbers lack.
I have regularly attended a meeting where they announce it as "one month" "two months". I think for reasons of simplicity, you could just count your "month" as the 23rd every month.
A healthy person wouldn't quibble with that.
That was a truly bizarre exchange, not sure I've ever witnessed that at a meeting.