Explanation:

A CANCELLED message is produced as soon as some alert criteria is changed or disabled using the Monitor Options page. At that point the alert testing process "starts over"; it doesn't matter what the change is. For example, even if the new alert condition is already met when the change is made, if there is a waiting period it is honored before a new alert is issued.

Also, if sampling is stopped while an Alert is active, the Alert is cancelled.

The "Recent alert history" section of a CANCELLED email is a list of recent ALERT, ALL CLEAR and CANCELLED conditions for this target database.

See also...
Help - Monitor Options
ALERT ...
ALL CLEAR ...