Why do moms always favorite their sons over their daughters ?

This could also be a youngest sibling thing. However, I do feel like it partly has to do with the fact that he's a boy. It seems my brother is the favorite between my sister and I. He's spoiled rotten and can throw fits to get what he wants. He gets away with way more than I did at his age. His laziness is astronomical. She never taught him responsibility like she did us. Not to mention manners, respect, accountability, etc. Whatever he does or doesn't do that she doesn't approve of, she gets mad at me or my sister instead. It drives me nuts. I can't help but feel like daughters tend to have to uphold standards that sons don't.

Edit : I understand what I wrote was a generalization. I wish I could edit the title. It was written quickly. I don't truly believe this favoritism is always the case in every family dynamic. However, it seems to happen often enough to strike a conversation.