That's the problem with most cooler ratings & comparisons. Too many variables for that claim to mean much, and it means nothing when comparing different coolers.
Kyle, you didn't mention doing this, but pre-chilling or freezing as much of the cooler contents as possible can make an appreciable difference. Also, having a second, smaller cooler for frequent access to drinks allows the main cooler to stay closed more, which should also improve endurance. Sun exposure also hurts, so throwing a light coloured towel over a cooler can help, and making a
Reflectix boot is even better. Unless you need crushed ice, I'd give up on it and stick with solid. Crushed ice takes up too much space if you're trying to stretch endurance as long as possible.
I hope that helps, and maybe someone will offer other suggestions. Good luck!