The numbers between each other don't mean anything, because of all of the variables. Weight, disk/drum brakes, truck manufacture, all impact where the number gets set. The brakes should be set per the procedure in the manual and whatever that number is, is what it is.
When we took our Montana in for trade, it was empty. After towing for several years at 7, that empty trailer would lay rubber at every stop sign until I set it down to 4.5. Also I put electric brakes on our boat trailer, and you should see what 6.5 does to an empty boat trailer heading down the ramp to retrieve the boat. I told Dory that trailer left half the tires on the ramp!
Set the brakes proper, then use the number as your guide and forget what everyone else does because it doesn't matter.