With copies of violins measurements are made and the new violin is made from thoughs measurements.  As the master luthiers work when carving out a top they tap on the wood hearing different tones from the wood.  So when one top is at one measurement another is at a different measurement.  It is these differences from one to another and the ability to match the carving of the wood to it's tone quality that makes each violin different.  There are differences in finishes also which effect the quality of the sound.  I saw video where one luthier puts his tops over a speaker, puts fine sands in it and at different frequencies the sands make different patterns.  By removing slight amounts of wood the patterns change and in that way he tunes his tops.

I wonder if anyone really knows the reasons for "great" violins EXCEPT for the meticulous repetition of construction details/materials that have been established by strictly empirical means. 

I can imagine a time when many village luthiers were making violins and one violin was different.  Much better.  Now what?  COPY IT!  Copy what?  Everything!  (Meticulous attention to detail is what happens for the lack of tangible principles.)

In any case, it is encouraging to hear reports that modern violins are being made which are as much world class as any Strad and I'll let the luthiers worry about how that is done