hahaha we are talking about coordinate systems.
each coordinate system has a geodetic datum, which is the reference used for positioning the earth when locations were collected. Most commonly used is NAD83, which is based on the geodetic reference system of 1980. NAD27 is a totally different ellipse. If you were to plot the same points using each datum, they would not line up exactly, there would be an offset to take into account that change in earth reference position.
fun stuff!