I can't imagine the disappointment of an empty bag...
I don't think there would be any empty bags or bags where the retail total was less than $390.
I think Apple is making sure these bags are filled with at least $390 (likely more) worth of Apple products; so even the worst bags are worth the $390 price.
My guess is every bag's content has items that in total is worth more than $390 retail if they were bought separately (making them in such high demand); but a few random bags has content worth a lot more than $390, such as the MacBook Air, etc. If I were to take another guess, likely about $490 worth of Apple products from last generation as a minimal. So everyone feels they have at least 20% off -- only they do not get to pick the product itself.