When looking at the Differences tab in the Version Changes tool to review differences between two database versions, it appears that differences are only assessed on current layers within the map.
What this effectively means is that I could edit two layers, A and B, within the same version. In this hypothetical, layer A is in an active ArcPro map and layer B has also been edited but is not currently in the ArcPro map. If I were to then assess the changes between my two versions, the differences tab would only show me differences on layer A (because it's currently in the map) and I'd be none the wiser that any edits were made to layer B. However, I assume that when a version is Reconciled/Posted, all changes to both layers A and B would be pushed through.
When using the differences tab to review changes in the version, the entire geodatabase should be assessed for changes, independently of whether the layers are in the map. Otherwise, users could be committing changes to a layer that they are unaware of!