This article describes A to C flights as being more expensive since they have a higher demand. Another practice they use is to discount smaller airports. It’s often cheaper to fly from A to B to C than it is to fly from B to C (on the same plane) if A is a small airport. I’ve asked when booking if i could miss my first flight at A and just go through security at B but I was told I wouldn’t be able to retain my ticket if I missed my first flight at A.