The "why" from the buyer's perspective is easy: tickets are cheaper bought from AA because they don't impose so many fuel surcharges.

From the seller's perspective: it's up to the individual airline office (or GSA) to determine whether they are happy to issue the ticket. Presumably the GSA in Manila gets enough commission that they are happy to do so. x% of a OWE price is substantially more money in real terms in Manila than it is in, say, New York.