I'll try this from a bit earlier point of view. In NEC Ch.9 table 8, coated vs. uncoated refers to tin plating over copper. The difference in resistance comes from both types having the same OD, but tin is less conductive than copper. So why the tin in the first place? Used to be that rubber (like in RHH/RHW) had some sulfur compounds that would corrode bare copper. Tin would limit that problem. With modern polymers the sulfur is no longer of concern, but some wire like SIS is still tin plated.
[Table 430-149 for 2-phase motors still...1999 anyways.]
