The number of open and resolved cases does not match

 The widgets show a different amount of cases because they are based on different data.

1. The Sum of Opened cases will show you the total number of opened cases within a time frame.
2. The other widgets filter out only Resolved and Closed cases
   

The amount may vary as, for example, if you opened a case on the September 30, it will be shown on the Sum of Opened Cases for the month of September but if you resolved on the October 1, it will be shown on the Sum of Resolved Cases for October, and in the Average FRT widgets, etc. 

In case you had a bigger amount of cases in the last days of October and only closed them in November, they will not appear in the reports of October but in the November ones, and so on.