Its surprising though Pete, even after all my years, yes i can slip under my regular tank spot on every time (though still check the height unless i dropped it) but all tanks are slightly different, and despite the height being right, every now and again on a strange tank especially another make if i’d carried on regardless the pin could easily have slid up one side of the fifth wheel plate missing the jaws completely.
I’m always annoyed with meself thinking how the hell after donkeys years can i be so far out 
As you rightly say a curtain or box is a different kettle of fish altogether.