Marshmallow, SD Card & Apps - Almost Nothing Movable?
(Summarising into an answer as suggested by Izzy.) 

When you adopt all or a partition of an SD card under Marshmallow (6.0) onwards, that SD card or partition becomes an extension of the internal memory. Depending on the phone model it may subsequently be referred to as internal memory, or it might still appear to be a completely separate entity. You can rename the card if you wish, to help clarify this. 

Android will install / move apps and data (as older versions of the OS referred to the parts) onto the SD card portion as it sees fit, and thus when it runs out of room in the internal memory, it will happily go on to use the SD card. 

When the internal memory or your card refers to 'Migrate data', this appears to be distinctly different from the data stored by any particular app, but instead appears to refer to that portion of the filing system that a connected device (PC or Mac) or a file manager app can view. Thus, there appears to be no value in using Migrate Data - just leave it where it is, the system will handle it. 

(By way of demonstration, as per my original question using Migrate Data from the internal storage appeared to only move about 1 Mb of data; when I migrated it back, the system suddenly decided to move 1 Gb's worth of app and data onto the card, thus showing that it was indeed moving items around intelligently.)