If a thread is not linear it may be counted as unresolved even if the last comment in the thread is resolved.
For example, if a comment thread has four comments ..
Comment 1: thread root, unresolved,
Comment 2: in reply to comment 1, unresolved,
Comment 3: also in reply to comment 1, unresolved,
Comment 4: in reply to comment 3, resolved
.. then the thread appears as resolved in the Gerrit UI because the final comment in the thread is marked as resolved.
However, both the `gr-comment-api#computeUnresolvedNum` method (which shows the counter in the file list rows) and the `ChangeData.java#unresolvedCommentCount` method (which determines the value of `unresolved_comment_count` on change details) count unresolved comments as the number of unresolved thread leaves.
Because comment 2 in this situation is a leaf (and in the UI only the final comment can be replied to) this makes the thread unresolved and unresolvable by these two measures.