Probably “number of users who were active this month” for monthly, versus “number of users who were active the last half year (6 mo)” for halfyear. Both can be updated monthly. A user who was active 3 months ago would be in the second group but not the first.

Well, are you trying to look at your subscribed communities without being logged in? How would that work?

Yeah you need to go to the settings of a specific account, not the general settings!