The average human body temperature is 97.9 °F, not 98.6 °F, because someone rounded before converting their units.
In the US, normal human body temperature is often said to be 98.6F degrees. In other places, like Russia, the commonly quoted number is 36.6 celcius, which is 97.9F. Why the difference? Both numbers come from the same source, in which a German study found that the average body temperature was 36.6 degrees celsius.
Because the proper number varies from person to person, the number was rounded up to 37 degrees, which gave the value of 98.6. If not rounded, it gives the value of 97.9. So which is right? In a sense, neither.
There is no absolute perfect body temperature. It varies from person to person, as well as the place on or in the body where temperature is taken. Oral temperatures are slightly lower than rectal ones, and skin temperatures are generally lower than oral ones. 98.6 is a commonly accepted average, but the average temperature for under the tongue is 98.2.
Still, these are just averages, and the healthy number is different for every person. Generally, each number varies about half a degree.