You're making a fundamental error.
You are assuming that air fares are logical.
They are not.
But someone has to program the computer, and they are usually given some guidelines as to what they are trying to achieve.
I would grant you that in the case of BA the guideline may be to make the fares as obscure as possible.
However, given that this is an anonymous forum, there always might be someone who is prepared to elucidate.