Thats cause your looking at it the wrong way. Keys in this era of CGC 9.8 are the following:
First appearance
First cover
We see it in back issues, it's why Batman 475 has jumped to ridiculous levels in the past 3 years since even though it is not the first appearance of Scarface, it is his first Cover appearance.
So think of it the same way. Venom 3 first print is the first appearance of Knull, but Venom 3 3rd print is his first cover appearance. So even though its the same issue, you now have 2 collectibles spawned from it and they are collectible in their own right. The fact that one is a 3rd print is inconsequential. We already saw this with Ms Marvel and how a 2nd print became super collectible because it was her first cover appearance.