Availability on MS Exchange server

Availability on MS Exchange server

Hello,
I have 3 monitors on a a single MS Exchange server. One for each role (Mailbox, Hub, Client Access). While checking the availability of each monitor. The mailbox one is 13% (for a month) et the others are 99%.
Is it possible? What does it means? I suppose that the server itself is available but something went wrong on the mailbox role (a service down?) C
Could you help me on understand on this case?

Thanks in advance
Fabrice

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