Why are parents abusive, even though they love you?

Both my parents, especially my dad, were emotionally abusive to me as a child. They screamed at me, told me I'm not as good as my sister, punished me about small things. My mom would literally scream at me and tell me I'm childish if I cried. My dad said because I wasn't good at Math, I won't get to study something worthy. Both my parents screamed so bad at me every morning before school if I was late, and even if there was the slightest possibility that I could be late. It got to the point where now, as an adult, when I realise I'm going to be late, I get panic attacks - I shake, I cry, I sweat. My dad only got physically abusive to me once. I refused to stay at home from school when I was sick, so he stormed me, slapped me across the face, threw me over the couch and ripped my school uniform off.
But they love me. I can see it in old pictures and in their behaviour sometimes. I don't think they love me as much as they love my sister, but still. Why would they do this stuff to me if they loved me?