I'd always just assumed it was something to do with where the internet payment was processed or the taxes and fees (figured BA had a global processing centre for it's e-commerce and it was based there), but having read the other thread linked below, I have to admit to feeling a bit sheepish now (no offence to Jenbel's Sheep intended :-) ), as RDM for Redemption makes much more sense, as does checkmytrip mistaking this as an airport code and displaying Redmond.