Weird. I just priced a dummy itinerary from IAH-NAS for April 15-22 on the direct flight. Prices for all U fares were:

1 Adults (age 18 to 64) $426.00
1 Infants (under age 2 at time of travel) $45.00
Additional Taxes/Fees $127.60
Total Price $598.60

So it's unclear how the 10% is applied. It's not 10% of adult fare (pre tax) and unless tax on adult fare is $24 (making lap child tax = $103.6) I don't see where the 10% is coming from.

Similar thing happens when I price out a Y fare:

1 Adults (age 18 to 64) $1,691.00
1 Infants (under age 2 at time of travel) $172.00
Additional Taxes/Fees $127.60
Total Price $1,990.60

Can anyone shed some light on my confusion?