It's pretty well established there's variation in wire thickness, and Duncans are definitely wound to a specified number of turns. I don't think wire tolerances could account for that much difference; IMO there has to be something else going on.
My oldest JB reads lowest- it's an '80s JBJ that measured 15.74. I have a 90s one and an 00s one that read 16.2 and 16.4; output-wise all three are right in the same ball park. The old one sounds better to me too but that may be due more to the magnet (rough A5 vs smooth?) than the coils themselves. Or not, maybe it's the MJ magic.
Anyway, somehow I feel it's unlikely that mere differences in wire could account for such a high reading. Maybe the OP's was a custom wind that wasn't marked?