Yes, Globaliser, I read what you said, I was obviously mistaken when I assumed an understanding of how simple statistics work.
You appear to like to judge the relative costs of airline tickets by looking at a single trip. When I look at a 7 day period I am actually looking at all 28 possible trips. If I took a 31 day period I would be looking at all 496 possible trips. So I am doing exactly the same as you are, but just more of them. Then I take all the possible trips and apply real simple statistics to them to give a rough idea of relative costs. Most people would say the more data points the better, so most people would say looking at a long period is better than looking at a short period.
Whan you took your single data point you said BA was 'only' £40 more and that is completely correct, for that trip. It was also £40 more than £40.
Look at it another way. There is a petrol station selling unleaded for £1 a litre, and it takes 40 litres to fill your tank. Cost £40.
Across the road is another petrol station and it charges £2 a litre for unleaded, cost to fill your tank is £80. It's only £40 more, but which petrol station would you use?
Ah you say, but if I pay £2 a litre I get a free sandwich. Well it's your money and you can do with it what you wish. I would pay the £1 a litre and buy a nicer sandwich at Marks and Spencer.