The revisions listed in the interdiff selector are not sorted.  Sometimes, databases will return things in insert-order, but not always.

One way I was able to reliably reproduce this is to:
1. Post review.
2. Click Update Attached Diff, upload a new patch.
3. Before clicking Save, click "Update Attached Diff" again.  This effectively deletes the 2nd patch.
4. Click Save.
5. Click "Update Attached Diff" and upload a 4th patch.

Checked the interdiff selector before and after my change and it is now sorted correctly.