This is largely because in the days when a large proportion of the population could neither read nor write, the census compiler would enter their names phonetically.
Often as you search further back through the censuses, surnames can get corrupted from those of today. This is largely because in the days when a large proportion of the population could neither read nor write, the census compiler would enter their names phonetically. Similarly for the forenames; quite often a child’s baptismal name may not be that by which the child is known to family and friends, and so may be recorded differently on the census.