Thanks for flagging this, and you’re right that the image is misleading. I’ll either update it or replace it, and add further details to the text to make this clearer. The answer actually depends on where the snap stores its data.
If the data is in a common directory (SNAP_USER_COMMON or SNAP_COMMON for the snap developer) then there’s only one instance of the data available across all revisions of the snap, regardless of any revision-specific modifications.
If it’s within a revision-specific folder (SNAP_DATA and SNAP_USER_DATA for the snap developer), then the diagram is correct - data associated with a revision normally migrates forwards, but not backwards after a revert.
Additionally, there’s also data saved in a Snapshot.
This is covered in Data locations, but I agree, it should be simplified and rewritten here.