Apparently I don't understand how posts work. I was told the game adds posts from the outside in. In a 9 man race there are two 8s. In a 10 man race there are two 8s and two 7s. ETC all the way to 2 each at 16 man race, then it repeats, and an 18 man race would have 3 8s and 8 7s
Slot #13 should be a 7th post in an 18 man race, but I guess lucky 7 requires you to be the actual slot #7, not post block 7. That makes it dramatically worse than I thought.