I've only skimmed this thread, so forgive me if I'm completely off target. What I would add is simply that there is a big difference between a max effort deadlift and a max effort clean deadlift (not to mention snatch DL). With the former, anything goes - leading with the hips, rounded back, whatever - essentially all that matters is getting the bar up. With a deadlift used specifically for weightlifting, positioning is critical, and loads that exceed your ability to maintain correct pulling posture (as defined for the clean and snatch, not DL) are defeating the purpose. It's surprisingly common to find people with 400+ lb deadlifts who can even clean 200 lbs, and often who can't even maintain back extension during the pull.
Additionally, the more advanced a weightlifter, the greater % of strength he or she will be able to apply to the lifts; that is, the gap between deadlift and clean/snatch will diminish. This is not an issue of less strength, but of better application. When technique is the limiting factor, the numbers can seem far off for strength work.