For the last dozen years, every major company has 'leveraged' (horrible word) computerised pricing.
Amongst the first were the mobile phone companies with a dizzing list of options all designed so that you could NOT compare like with like across their competitors. The energy companies enthusiastically followed suit.
As to the amusing case you quote: I think it's rare that the gap is as wide as this but one of the reasons might be - if one or other of the legs has some kind of a promotion on it to attract more traffic, that would show up more in a simple return, than in the full journey.
But speculation is useless. I doubt that anyone in BA's pricing team could tell you either - because it is the computer that is pricing this (within boundaries) and no human is involved. I agree with Tableview, book the cheapest and for the return, mebbe schedule a night stop with friends too? Otherwise, allow extra time if the long haul is delayed. For me, the tipping point is about £100 saving.