The 'Mailbox Location Checker' scheduled task performs the mailbox count. This value is used until the scheduled task runs again and updates the count. Active Roles does not check dynamically during mailbox creation.
When using the Round Robin method for the selection of mailbox stores policy, Active Roles keeps track of the last store it used and then moves onto the next - the source list of mailbox stores being what is found at the default 2 am run to gather the list of Exchange servers. The list also does not get updated until the scheduled task runs again or if the Active Roles service restarts.