I don't find any document to explain specifically which change sets get compared in Pending Changes view.

Below is my testing result when the same file is present in Incoming, Outgoing and Unresolved folder:
- double click the file in Unresolved, the left side of Comparing tool is the content in your local sandbox, the right side is the latest change set in the workspace.
- double click the file in a completed change set of Outgoing folder, the left side shows the content you modified in the outgoing change set; the right side shows the content of the predecessor change set.
- double click the file in an active change set of the Outgoing folder, the left side is the content in your local sandbox, the right side shows the content of the predecessor change set.
- double click the file in a change set of Incoming folder, the left side shows the content in the incoming change set; the right side shows the content of the predecessor change set of the incoming change set.

To find the change set's predecessor change set, you can right-click the file in Pending Changes view and select "Show history". Open the file's history in Outgoing show its history in the workspace. Open the file's history in Incoming show its history on the stream.

Hope the information is helpful.