Generally what fuel your body burns depends on the intensity of work you are doing, so if you're walking you may be burning 80%+ fat, but if you're running a 10k you may be 80% glycogen.
However, if you eat too many carbs at a meal (beyond what you need in the next few hours), your body needs to get that excess out of your bloodstream, so it will do a combination of: - fill your muscle/liver glycogen stores (limited size) - burn any excess carbs in preferences to stored fat - convert any excess carbs to stored fat
Once it's got rid of the excess carbs from your bloodstream, it will return to burning whatever the normal ratio of fat:carbs is for you.
Consequently, if you always eat too much carbs, you'll gradually pile on the fat, unless you're doing large amounts of training which depletes your muscle/liver glycogen, e.g: elite athletes.
I was always taught that the body uses up its glycogen stores before burning fat. But if that were true even a small calorie deficit would gradually use up our glycogen stores and we'd be walking around without any after that.
Plus before losing any fat you'd have to experience a glycogen crash/wall which most small deficit dieters don't experience.