An entry in a Parish Burial Register is just a recording of the fact that a burial took place.
Where it took place is not always recorded, though in the vast majority of cases it took place in the graveyard of the Parish Church and in later years the grave or plot number is recorded.
Sometimes the entry indicates that the actual burial occured at another place.

MIs are another problem as the majority of cases they are erected by survivors - occaisionally instructions and funds were provided before death.
It is probably reasonable to assume that the first name on an MI was actually buried at that spot but subsequent names are problamatical.
Were infants and young children actually buried in the same grave as elderly parents or is an MI just preserving their memory?
Much will depend on how long the family owned the plot.

Also, even though the data are "set in stone" they are not always correct.