I have heard of many ppl complaining how they ordered overnight and it came later than those who ordered standard
They fulfill the orders in the order they came in.
So should you say the standard shipping came in at order number 5, and the overnight order came in at #23,000 then of course the overnight will come later. As it was only processed 3 days later and shipped the next day with overnight shipping or whatever.
It's the shipping speed, not the fulfillment speed.