Not only is it a matter of perception, but also expectations (if you expect high crowds and get medium crowds, they will seem low; if you expect low crowds and get medium crowds, they will seem high).
And sometimes it's just plain good or bad luck. Sometimes even the best planner hits a string of rides or areas at just the wrong time, and therefore would rightfully conclude the park is crowded. (Never mind the poor folks who arrive a few hours after opening and go right to RSR!)
Someone with better luck (or sometimes, better planning) might go the same day and think the park wasn't so bad.