Something does appear strange, but is explainable. ADL-HKG-NKG is 4,996 miles, so 80,000 Asia Miles Business return (award zone c, 2501-5000 miles). The redemption would be quoted on the total distance (a single return), not two separate returns ADL-HKG (4,277 miles - award zone c, 2501-5000 miles) 80,000 Asia Miles and HKG-NKG (719 miles, award zone b 601 - 1200 miles) 40,000 Asia Miles. This relies on the distance used by Asia Miles / Cathay etc. for the actual legs.
To get the 80,000 redemption, it would need to be booked as a single return, which is what you are attempting. You should be allowed stopovers as well (but would have to go via the call centre to do that).
The actual redemption is quoting the 120,000, or an Award zone D, 5,001- 7,500 miles (not the original amount shown). I suspect the problem is the distance - 4,996 miles (from mileage monkey), for the two sectors is probably not be the figures Asia Miles is using - it only needs 5 miles more, to push it over. The initial 80,000 coming up is because you have requested an ADL-NKG, and the lowest cost would be based on a direct flight - distance ADL-NKG being 4,786 miles, so clearly Zone C and 80,000. When actual flights are located, there are no direct flights, and routing via HKG is pushing the distance over the 5,000. It is just coincidental that the Zone D amount of 120,000 (being the next longest) is actually the same as 80,000 + 40,000 which would actually apply if booking the two legs as separate return redemptions.
If you call the service desk, they can probably tell you the distances being used for ADL-HKG and HKG-NKG so that you can confirm what is happening.
You don't need to have the miles to get to the booking page - just fiddle with dates until you get options that are FULL but Waitlist is available - which will let you get to the final screen.
