The precise count of bones can vary among individuals because of natural anatomical variations.
As a person ages, some bones fuse, a process which typically lasts until sometime within the third decade of life.
Individuals may have more or fewer bones than the average (even accounting for developmental stage) owing to anatomical variations.
Other genetic conditions may result in abnormally higher (e.g. polydactyly or conjoined twins) or lower (e.g. oligodactyly) counts of bones.
Note that the posterior bones comprise the lower parts of the spine, where they are primarily counted to.