Where has the idea that manufacturers performance data is unachieveable come from? It's absolute rubbish to state that an average line pilot cannot achieve book figures. A manufacturer would be crucified in court if book figures were not achievable. Crossing the threshold at 50 feet on speed, closing the throttles, and performing a normal landing with maximum braking will produce at least the book figures and if a line pilot can't do that then he or she needs to find another job!