Can someone please help me better understand the tax charges?

I'm doing a HKG->SIN->KUL->HKG trip using Asiamiles.

The reward booking total taxes told to me by the agent is HKD 948.

I did calculations using $ purchase of tickets:
- HK->SIN = HKD 255
- SIN->KL = SGD34
- KL->HKG = MYR 138.38

Total when converter all back to HKD = 255+200+274.97 = HKD 729.97
Anyone explain to me why there is approx HKD 218.03 difference?