When performing the following query:

status = Open AND status != Closed

The results are all the issues in "Open" status. The second clause of the query only retrieves issues that are not closed, but since the first clause only retrieves opened issues and the AND operator is used to join then, only Open issues are retrieved.

However, using the same analogy with the resolution field, the results are different. When performing the following query:

resolution = Unresolved and resolution != Fixed

No results are retrieved. I believe both queries should behave in the same way.