We are all different with widely varying physiology. People have varying tolerances for many different foods, different allergies, different metabolisms, different genetics. One person can eat the very same foods and calorie content with the same general activity level as the other person, but one will gain excessive weight while the other will not. One person loses weight fairly easily with diet and exercise while the other doesn't have that great experience and is frustrated.
Potatoes make you sleepy for instance while they do not have that effect on most people.
That's why we can teach nutrition etc. on a general level--everybody needs protein, fat, citrus, vitamins/minerals etc.--but nobody knows the exact specific needs or intolerances of another person. For all of us, it is generally a matter of random trial and error that informs us.
So keep giving good advice with the caveat acknowledging that it might not be good advice for everybody.