First thing to do is to ascertain if they really are different obverses: easily done - 13 has the "missing leaf" compared to 12. If so, no need to worry about fractional possible differences around nose/lips/eye area. If they prove to be the same obverse, like Richard and Rob, I don't think there's any significant difference worth noting. Could well be depth/pressure of strike, wear of die, resistance of metal in each blank etc.
With the 1899 and other date widths you might see, you really need to get hold of that latest Gouby (2020 additions) as he covers all manner of subtle date varieties - but as we've said before on these threads, many other date widths (and heights!!) are known, even if not catalogued by Gouby yet.