I don't understand my parents logic, help?

I'm really frustrated. My mothers spends thousands on clothing and trinkets for herself. For mothers day, she received an expensive purse, and a few weeks later she bought my father an equally expensive watch. However, if I try to buy something like more pens for school, she will throw an extreme fit and start yelling and screaming. I really cannot understand. What is her logic? Is it because she believes she is the 'boss' of the house? I'm the primary caretaker of my brother and sister, I watch them all day while my father either works or shops with my mother, or watches tv. I'm just really confounded by their behavior, and I can't think of any stressors that wound trigger it.