If the spike were driven in securely and held firmly, the dimple would retain the shape of the tool. If the grip of the spike was weaker than the force of the grinding tool, the coin could rotate on the point producing a perfectly round dimple. I do not claim this answer is without problems but, for the present, it seems the best of the choices. This question bothers me greatly. It would seem that the same minds that thought it desirable to smooth the flan surfaces at all would have been terribly disturbed by the pits that remained.
Weight control seems an unlikely reason since our three coins vary greatly from each other.
This is simply a subject that requires more study. If you have evidence on the matter, please e-mail me . In fact there are several other points of interest that will speak to different groups of people. Those interested in epigraphy may note the ligate NK, NT, V G and two different ligatures for Omega-Nu. Unfortunately the off center strikes of the other coins makes this only seen on the Demeter.