I've been doing this for years. After I noticed most places are cheap to get to but flights are expensive back. I noticed the same business model was in place from the other direction. So just fake the country of origin and you get cheap to and back. The other one is clear your cookies before booking. Places create fake demand by disappearing the cheap flights on you if you shopping around. If you clear your cookies…
So you book two one-way tickets? AFAICT, that's usually not cheaper.
But if you're in Croatia you can get flights to London $200 etc... geofenced deals marketing deals to try and coerce people into taking holidays.