'Reserve Heads' are another aspect of funerary goods not yet fully understood. One thing I do think they tell us is that funerary practices in ancient Egypt constantly evolved and were sometimes very specific to locations. Comparatively few have been found and in a limited time period.
In cases like this it is often helpful to look at a wider picture: during this same period, changes to the designs of mastabas and their locations was also occurring.(Reisner 1934)