A coil I built has divergent measurements respective to wire resistance and AWG size
The above (modified) picture came from this wiki site on magnet wire. it suggests that the overall diameter is made up from about 8% insulation. The insulation is 0.015 mm overall. That site also states this: -


  Breakdown voltage depends on the thickness of the covering, which can
  be of 3 types: Grade 1, Grade 2 and Grade 3. Higher grades have
  thicker insulation and thus higher breakdown voltages


So, do you know how thick that insulation is?


  If all of this is enamel and this is AWG 29 wire, than the enamel
  layer is 0.016mm thick


OK, so assuming it is insulation grade 1, that explains half the thickness of varnish but, varnishes come in all sizes (as hinted above) and who is to say that the varnish on your wire isn't a little thicker than the "typical" grade 1 standard? I mean, where did the wire come from? Does it have any provenance?