What's odd about that? Every single TUI branded flight from anywhere in Europe to Heraklion over the past 8 days would've been a ferry, it's the end of the season. Operating ABZ-HER-HAJ saved an extra ferry sector and allowed them to reposition the crew back to Germany efficiently. Not odd at all.
As for which aircraft go to Sunwing, this will be dictated largely by maintenance cycles and will be aircraft that don't need to visit a hangar for 3-4 months. Maintenance is obviously pre-planned months in advance, so they'll know which aircraft are going and plan accordingly. Whilst stickers can be added and removed fairly quickly and cheaply, repainting a whole tail costs money and isn't a priority during 18 months of little income.