Are you referring to the suggestion header?
That’s saying you have an open (un-merged or un-closed) suggestion, and the “View changes” link will compare your fork to its parent.
Or are you referring to the unpublished (or unshared) changes header?
If you’re seeing that, it means you’ve edited the notebook since you last published or shared. You can review your changes by clicking the “View changes” link or, more explicitly, by clicking History in the notebook menu, which will show you something like:
If you clicked “View changes”, the compare may still show no differences if you’ve edited and then manually reverted those changes (e.g., pinning and then unpinning a cell, as above). Also, the compare view currently doesn’t make it clear when the changes involving pinning or moving cells; it only shows additions, modifications, and removals.
We’d like the unpublished changes message to be based on a content hash rather than the version number, but we haven’t implemented this yet. We’re also hoping to find a way to have the compare view reveal these other types of changes.