Maybe IBERIA have a fixed allocation of seats on each BA flight and they've more unsold seats at present than BA on the flights you want. Strange really, because generally I find IBE more expensive than anyone else whenever I want to travel to Spain (not from UK I hasten to add). I often see the same sort of thing on Brussels Airlines/British Airways flights.
Last year, booking some two months in advance, IBE wanted 1500 Euros for an ECONOMY round trip from DUS to MAD and for the same dates LTU asked for 228 Euros in their BUSINESS CLASS! Guess who I flew with. It's called shopping around as we do for most other products too.