We know the Earth is ellipsoidal-ish and really rather bumpy which is why we work with and have to understand many projections, datums and all their varying components. On top of this it is important to know that the Earth's surface is not static and in fact it is continuously changing shape (e.g. tectonic movement, rotation etc). This is one of the reasons why we see new datums being released.
Introducing the dynamic coordinate reference system - to help introduce this concept the IOGP Geomatics committee released this video.
And find out additional information at the 2017 European Petroleum User Group Conference.