By current design. Active Roles counts additional mailbox types such as System, Arbitration, and Archive. When taking all of these mailbox types into account, the numbers are correct when Active Roles chooses the "least" number of mailboxes in its own count. However, the end result will differ when comparing in native tools such as Get-Mailbox which do not count additional mailbox types.
EXAMPLE
Active Roles count:
Mbox Exch1 = 43
Mbox Exch2 = 19
Mbox Exch3 = 18
Mbox Exch4 = 16
Get-Mailbox count:
Mbox Exch1 = 12
Mbox Exch2 = 3
Mbox Exch3 = 5
Mbox Exch4 = 7
When Active Roles considers the least number of mailboxes, it will choose Exch4, whereas native numbers indicate it should be Exch2. Active Roles is correct when it's comparing its own numbers to the other mailbox stores, but not when comparing to native tools which only count User mailbox types.
Defect 437489 has been logged to change the behavior so that Active Roles only counts User mailbox types to match native tools such as Exchange Administrative Center and Get-Mailbox.
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