Maybe being #1 Club Gold Member (I'm a Five Star member) has something to do with it. Consider that since #1 Club Gold members are typically frequent renters, they (Express checkout agents or counter agents) may think that they "slip" a charge in unnoticed....
The leisure renter would likely be more careful to check their receipt-as it is coming out of their own pockets, versus the biz traveller who may expense their rental on a Company corporate card....
Just a thought.