The reason is that the conversion will depend on how much water, fiber, and other non-caloric ingredients are in the item. For instance, you could add a drop of oil to a glass of water. It is better to use the fat formula from section 3.1 if at all possible. This is how foods can have oil or high-fat ingredients listed in their ingredients yet claim to have only 0 grams of fat. The fat we are most familiar with is the triglyceride.
All oils are triglycerides.
However, there are also other forms of fat that you will see in ingredient lists. These are fats just like triglycerides and also have 9 calories per gram. Lecithin metabolism is somewhat different. These books give nutritional info for a variety of fresh and packaged foods. While you can ask on this group about the fat content of various foods, it is considered polite to first attempt to look up the info yourself.