98.6°F is an over-precise conversion of 37°C. The number was originally measured as 37°C, with two digits of precision. It shouldn't have been reported as 98.6°F, though a correct rounding would have been even less accurate: the number is closer to 98.2°F/36.8°C, but the proper rounding of the conversion would have rounded up to 99°F.

There is, of course, significant variation from person to person, so two digits of precision is probably all that's really appropriate.