Human Variation! There are many, many, MANY features of the skeleton that change between individuals. These could be confused with pathology, or trauma if the student does not see the differences between skeletons of different people.
How are skeletons so different between humans?
Think of how different every person you see daily is. Some are taller, shorter, some have bigger noses than others, some may be bodybuilders, some may have broken bones or had surgery. All these differences can be found in skeletons, but you need to study many different ones!
Why do professionals need to know how they differ?
It is important students learn what real bone looks like, often replicas lack the detail needed to learn the skeletal features to the extent needed to apply their skills in the field. Human osteology is a lot more than just knowing the names of every bone. Many osteologists need to know what bone they have found and what side of the body it is from even if it is in fragments!