Ah I see the problem... it is backtracking... and only for DEL-BKK! There is a sneaky asterisk in the award chart which goes on to say:
* Zonal combination includes certain backtracking routes and routes exceeding the
maximum permitted mileage between two zones, to which the reference fare will not
apply. To confirm the exact mileage required, contact KrisFlyer Membership Services
Grrrrrrrrr!!!!!!!