Originally Posted by mzkaiser

Look at it from the same dates, in similar fare classes.

I don't know where you found PHL-SIN for 1,000 unless it is highly discounted, I ran ORD-SFO and ORD-SIN on 12/22/06:

ORD-SIN
12/22/06
$1484 R/T QPXPXSG (lowest published fare)

ORD-SFO
12/22/06
$428 R/T SE14A1N (lowest published fare)

Sure, ORD-SIN is 16k miles and ORD-SFO is only 3.6k but that extra $1,000 times all those Y seats is incredible. Not to mention C and F that are anywhere from 4k-15k each. The routes are not only more expensive but often sold out, whereas ORD-SFO (or PHL-DFW) are almost never completely sold out everyday.

The extra gas mileage is offset by the number of people, and UA has hotel deals so that FAs can stay for next to nothing.

I'm sure other FTers can explain more, but that's kind of the idea...