difference in the ability to scheme is rooted in the characters of the two ladies, formed by their social positions: Lucy was always dependent and poor, so she had to practise much more scheming, manipulatiion and flattery; while Lady Middleton is the wife of a rich country gentleman and the daughter of a rich indulgent lady, so she was and is much less dependent, always had an enviable social position. Therefore Lady Middleton could get away without being really scheming, she just needs to flatter herself believing in her skills as a deft manipulator.

Of course, the ability to scheme depends on the innate affinity of the individual and here again Lucy is better equipped than Lady Middleton.