FlyerTalk Evangelist and Ambassador: The British Airways Club
Join Date: Jun 2008
Location: Somewhere between 0 and 13,000 metres high
Programs: AF/KL Life Plat, BA GGL+GfL, ALL Diam, Hilton Diam, Marriott Gold, blablablah, etc
Posts: 33,120
They don't. On many itineraries, you will find BA cheaper than KL, on others AZ will beat them all, or LH or TK or SN or whoever. It depends on many factors (where you go from, where you go to, when, when are you booking, which flights are busy or not, etc) which mean that you have simply found that on your specific itinerary/date/flights/time of booking combination BA is not the best value and you should just go KL.