Most airlines are keen to get pax who originate outside the country of the airline. So they have a bias towards cheaper flights. It is like flyer miles schemes - they are often (but not always) better for "foreigners".
In the past, the cheapest way for me to fly on BA to the USA was to buy LHR-BRU return, and then BRU-LHR-wherever return.
Daft, but that is how it used to work. BA were very keen to get folk from BRU to use LHR as a hub.
Even tho the Belgian airlines were not exactly competition!