Glynis, I would say based in my own experience that it is definitely linked to the shape of your feet. Some people have flip flop shoEd feet and some of us just don't!
I do usually have one basic rubber or plastic pair for use around the pools holiday because I don't want to ruin a good leather pair of sandals but I usually have to buy a size bigger than my standard shoe size. It's the only way to make us that the toe post isn't pulled really tight against the fleshy area that I have between my big toes and the neighbouring one because of the slight webbing. I'm dreading the time when the current pair (from Accessorize) give up the ghost because they are the only pair that I have ever had that I have been a ble to spend any time walking around in. One of my nieces has the same problem and she splashes out on Havianas but I'm too mean that spend that sort of money for the little time I actually wear flip flops for.
So it could well be the case that having a high instep has the same effect on you as the webbing has on us - it pulls the toe post in very tight to where your toes join and making chaffing more likely?
I do feel envious of the lovely bejewelled toe post heeled and wedge sandals that many of the Asian women I know wear when dressed to the nines. They look gorgeous but just the thought of putting them on makes me wince. I know that the way my foot would slide down the slope, so to speak, would put intolerable pressure on the area between my toes. It's bad enough with absolutely flat poolside ones but it would be agony doing anything but sitting around in them for me - I know without even trying that I wouldn't be able to walk in them.
SM