As I understand it, based on posts in the LH forum and from my own experience, LH reserves bulkheads and exit rows for day-of-check-in. Families with little one get bulkheads, exit rows are FCFS (first come, first served). However, many seasoned LH flyers get these seats through early (web) check-in or by having an angel at LH (I know that sounds hard to believe), or perhaps through more nefarious means?
As a 1P flying on LH you rate just a notch above "pond scum", no personal offense intended. I am a 1K and a LH M&M SEN so I am just two notches above pond scum.