That was exactly my association, too! But in fact it's the main wife, while the concubines are all "Little Madam".

It's either because he's the second son or because Minglan, for convoluted familial relationship reasons, originally referred to him as "Second Uncle" (rather than er-gege, although they aren't that far apart in age, at least by Ancient China standards). Minglan is having none of it and continues, as far as I can tell, to call him Marquis.