Everyone that is 3Q'd goes to the NWL. Then, Admissions determines the winners of each congressional slate, either by principal nomination or high WCS. So, that could be anywhere from ~500 slots to ~669 slots (if you assume the 535 MOC's has an average of 1.25 slots/year - but the 669 number is really too high because some MOC's don't nominate anybody and some end up not having anybody that is 3Q'd).
In any event, after those 550-600 MOC slate winner slots are determined, then "by law," Admissions must choose at least 150 more candidates that have Congressional nominations (but did not win their particular slate) and appoint them. Now, you're up to maybe 750 appointees (~600 + the 150 non-winners) that all had a congressional nomination. That's going to be over half the class. I assume Congress did this to maintain its authority in the admissions process. Otherwise, West Point could use Presidential nominations (service related - limited by law to an average of 100 slots per class) and Secretary of the Army nominations to appoint more than half the class. It's almost like a Constitutional separation of powers issue.