The gartel is a natural occurence, the string theory is a myth. When the esrog starts to bud, there is a ring of fibrils surrounding the developing fruit that exerts a certain amount of pressure on the midsection of the fruit. At a certain point, this ring falls away to allow the fruit to continue to develop. Sometimes, the ring does not fall off so quickly, and the pressure indents the fruit in the middle, causing the belt tightening effect we call the gartel. Sometimes it’s 360 degrees around and sometimes not. It is an anatomical quirk and is not related to the kashrus of the esrog. Every tree will have a few like that, some more, some less, it’s pretty random.
Some people like it, some don’t, the hiddur of it is dependent on what your ancestors liked or didn’t. To each his own.