When you purchase a Delta Airlines Zed from myIDtravel, Delta Airlines sends you an email with an itemized receipt. My last booking email looked like this
Total Tax: 9.83
Itemized Tax: 2.50 AY 3.80 ZP 3.53 US
When you book an AA Zed via Cathay's system for the same route, you are charged four taxes:
Tax Code Tax Amount
US 19 HKD ($2.44)
AY 20 HKD ($2.50)
ZP 30 HKD ($3.80)
XF 35 HKD ($4.50)
The "US" tax is 7.5% of the fare, that is why the Delta Zed is more (Zed Medium versus Zed Low). The AY and ZP taxes are fixed at $2.50 and $3.80.