At a company like Facebook, the difference between L4 and L6 is over $100k in salary, and $80k in stock (difference in 4 year refresh grant of $325k). $180k a year difference isn't almost nothing.
If there's a ~$180k difference between the average L6 and the average L4, and the bands are that wide - you have a lot of L4s making more than L6s.
If an L4 is "Exceeding Expectations" enough to get huge grants and bonuses, and an L4 is "Needing Improvement" enough to get terrible grants and bonuses... Why is the L4 an L4 and the L6 an L6?
Additionally, everyone has different (with big swings) of how much of their compensation comes from stock vs salary & bonus. When appreciation comes into play - this has large implications.
Additionally, people working between 2-4 years often make substantially more money than anyone else at the company in their current level.