Because there is some outside of the box thinking going on.

You book the trip as 2x one ways. It also gives you the flexibility to change the return leg if your plans change as well.

Anyway, basically what G_G is saying is that you do the outbound as a Z2-Z8 (CPH-NRT). You can then stop over in NRT as long as you need to, as that is the final destination on that ticket. As an aside, you'll save yourself a heap of APD in the process.

The the return is a Z8-Z1 starting in NRT. But because you will also have booked this as a one way, you can then have a stopover in BKK, again for as long as you need.

You can also then book a side trip to the beach, either as a separate award or as a paid ticket within the dates of your BKK stopover (or even a bus journey ). You then resume your journey in BKK for your final homeward journey back to MAN.

This is only possible by booking the trip as 2 one-ways. A return would only allow you a stopover in NRT for the duration of your stay.

Simples.