Less then 100% uptime when maintenance window is used - why?

[arminus]

I don't want to raise an issue yet because I'm not sure if this is by design or a bug.

Any monitor which uses a maintenance window never gets to 100% uptime, even if it was 100% up during the "non-maintenance" times. I find this misleading, < 100% uptime to me indicates a potential problem - which in this case isn't a problem ...