In a apples and oranges sort of way yes the same number size can mean a difference that large. What I mean is your size in say athletic type shoes might be different than the same number size of espedrille or stiletto. It seems the seller is trying to have one number represent several dimensions which doesn’t really help anything since it still means the bigger the number the bigger the shoe.

And yes absolutely those can be stretched. The tool we use is a vaguely foot shaped form usually made of wood with a large screw that when cranked down will separate the sections of the form to increase the volume it takes up. After I’ve put the shoe under tension I’ll wet the shoe down with a solution that allows the leather to become more pliable without staining anything. If you just need that one strap to give a bit more than it shouldn’t be too much of a problem but if its the wrong size it going to stay the wrong size no matter what you do.