Scuffy heels

I've noticed I have identical scuff marks on the heels of my high heels (grr). See how there's an exposed white bit?















I wonder how it is that I'm causing them. Is it because when I walk on soft grass, the heel sinks into the ground and when I pull it out, it gets scuffed? Or am I scraping my heels against concrete steps or something? Anyone know?

I'm kind of annoyed, but I'm sure not that many people are looking at the heels of my heels ('cept you now will, dear reader).