Because we only see the fault in others, not in ourselves and are blind even to our smallest defect. This most subtle form of this was noted by de la Rochfoucauld in his Maxims: 'The person who asks another to keep a secret cannot keep one himself'
Diesel cars stopped stinking years ago, cupid. Your OH just discovered it late, that's all !
My late father went to buy a camera in the '60s. He wanted a Leica. He was shown one and then offered a Pentax. He always swore blind he wouldn't buy anything from those Japanese b......s, after what they done to our boys in the War. Then he was told that the Japanese one compared well with the German one but was much cheaper. He bought the Pentax!