Why can prices vary wildly between two country versions of the same engine (for example, Expedia US vs JP, or Kayak US vs SG)? I sometimes even find that not all the airlines available in one are available in the other.
Both fare tariffs and flight availability (how busy a flight appears) vary based on the city in which a travel agent is based. This is called the “sales city” or the “point of sale”. This is simple revenue management practice; for instance a customer living in New York may be willing and able to pay more for a flight than a customer living in Santiago de Chile. Hence the airline must find a way to charge these two groups of customers different amounts.
The system was devised in the 1960s and 1970s when flights were almost always bought through a local sales office.
Many flight search engines and online travel agents base their sales city location on which country website was selected. For instance expedia.de uses a sales city in Germany whereas expedia.co.uk uses London. Other sites always use the origin city as the sales city.
This practice is illegal within the European Union under EC Regulation 1008/2008 Article 23 but it nonetheless continues there.