Unfortunately, that report isn't exactly what you think. At best, you divide by the number of days to get the average daily information, but that differs from "what users were active this month".
We only track the last time a user was active, so the only way to get a list of users (or aggregate number) of users active in a given month is to check if the last activity is within the current month (when checked at the end of the month). This would need to be stored then to track further information over time.