Is it possible that some people’s bones are heavier than others (thus explaining why some seemingly same-sized people would weigh different amounts) or should a person’s weight be wholly attributed to muscle and fat?
Yes, bones vary in density. Some people’s bones have more mineral in them and are heavier than others. Lack of mineral content in bones is termed osteoporosis - leading to easily fractured bones and debilitation.
However, the difference between the heaviest bone and lightest bone of any given size isn’t much - differences in muscle mass are probably responsible for 2 people of similar stature and size weighing significantly different amounts.