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All this seems pretty complicated.  Is there an explanation which will satisfy the man on the Clapham omnibus?  Here is my explanation.  It’s probably as neat, plausible, and wrong, as most of the simple 2-D explanations of how a wing produces lift, but I make it anyway for you to snipe bits off.
Neglecting the ‘flat Earthers’, it was long felt sufficient to assume the Earth was a perfect sphere.  Then it was realised that this wasn’t quite true.  It was actually slightly squashed at the poles; it was in fact an ellipsoid.  It wasn’t even a perfect ellipsoid; it was a lumpy ellipsoid.  Now to most people spherical geometry is hard to do, ellipsoidal geometry is very hard to do, and true Earth ellipsoidal geometry is dammed near impossible to do, so what did old time cartographers do in their quest to map there own counties as accurately as possible?  They chose the dimensions of the perfect ellipsoid which best fitted the surface of the real Earth where their country was situated.  Hence we see the growth of a large number of datums specifying the particular dimensions of perfect ellipsoids.
Now all this was fine until people needed to think in terms of the Earth as a whole.  E.g. programming ICBMs, and developing GNSS.  They had to choose an ellipsoid which best fitted the overall shape of the Earth.  This led to the development of the WGS Datums of which the 1984 version is currently used.  Therefore, it’s not surprising that places can have more than one lat and long depending on the datum used.  Is this another way of saying the same thing in simpler terms, or is it something completely different?  In bringing in the deflection of the vertical (which would have reulted in tiny differences in early astronomical measurements), Malys et al would appear to knock this argument flat, but does it?  Perhaps the lumpy ellipsoid is the cause of both the deflection of the vertical and the development of different ellipsoids to best describe different parts of the World.  Dave