As has been posted here before, this isn't something which EK made up. Fares ex every country are filed separately and approved by the government from the origin airport. The only fares that should price exactly the same in both directions (assuming that the fare class is available/applicable) are wholly domestic flights. This has been the case for decades. Thus for a while people were going out of their way to buy tickets originating in BKK when the fares were low from there in the mid 90s, then it shifted to CMB when J/F fares were low from there. TIP also had subsidized fares from there that were only available for tickets purchased there. Every country has its own restrictions and competitive environment. Currencies also rise and fall. Sometimes it can work out to be less expensive to buy two one ways if prices ex one country are significantly higher than fares ex another country. Sometimes bucket shops have lower fares ex one country than another based upon incentives to get passengers in that country on one airline or another.