The bones of St. Nicholas (or, at least, his purported relics) rest in the crypt of in Basilica di San Nicola in Bari, Italy. They've been disinterred, measured, and documented, and over the years various anatomists and forensic anthropologists have taken a stab at reconstructing what the real Santa might have looked like. The results vary widely. Why?
In 2010, Caroline Wilkinson of the Centre for Anatomy & Human Identification at the UK's University of Dundee wrote an easy-to-read (and publicly accessible) research paper about the flaws of facial reconstruction techniques — flaws that are exacerbated when all you have to go on are dry bones.
The skull is made up of 22 bones, 14 facial and eight cranial bones; it is a complex structure, and small variations during development and growth, together with soft tissue differences, create the enormous facial variation seen in the human population.
...Traditionally the nose has been considered a feature with poor levels of reconstruction accuracy and there have been many studies assessing the relationship between the configuration of the nasal tissue with the bones surrounding the nasal aperture ... The morphology of the mouth is an area of the face where there is more reliance on artistic interpretation ... Ear shape is also very difficult to determine ... The final stage of the facial reconstruction process is the addition of a skin layer (with subdermal connective and adipose tissue) over the muscle structure to fill the face out to the level of the tissue depth pegs. The tissue depth pegs represent the mean tissue at an anatomical point related to the ethnic group, sex and age of the individual. There is an assumption with the use of these datasets that the individual has an average amount of fat over the surface of the face. This may or may not be true, but since it is currently impossible to determine facial fatness from the skeletal structure, this assumption is necessary and based on the hope that a familiar face will be recognizable even where the reconstruction involves a reduction or gain of weight.
Basically, all of the soft stuff involves a decent amount of guesswork. There's a science to facial reconstruction. But it's definitely not as neat and tidy a science as you might see it portrayed in crime shows and Discovery Channel documentaries. The result: One set of bones equals many possible Santas.