When a case is resolved, the "resolved" date (resolved_at) is populated only the first time the case is marked resolved. If the case is re-opened, the "resolved" field does not get cleared and is not populated with the last date/time the case is ultimately resolved.
Out of the box expected behavior since ServiceNow has nothing to clear these fields when the case is reopened.
Attached is the out of box business rule to this incident. You can navigate the the following URL in one of your instances that you have access to. Just change "InstanceName" in the URL with your Instance.
Building a similar business rule for your customer service case table should resolve this issue.