Also you probably need to factor in sacks as lost yardage on QB runs. Let's say as an example he scrambles four times in a game, three times he is dropped for a sack, losing 12 yards
the 4th time he gets loose for a 20 yard gain. That gives him 4 scrambles for 8 yards with a 2 yard average. Rather than 1-20 with an average of 20. Hence, big chunks, from open field running balancing out.
I appreciate that's not the way the NFL record stats for QB scrambles but it does put it in perspective.