Why do temperatures vary between oral, rectal, or axillary measurement sites?

Core body temperature most closely approximates the temperature obtained through rectal measurement. Oral temperatures will generally run 1 degree lower than rectal temperatures. Axillary (under the arm) temperatures will generally run 1 degree below oral (2 below rectal). Remember, 98.6 is an average of the patient population. Your average body temperature might vary by up to 2 degrees from that.  And while not as volatile as blood pressure, your temperature will vary throughout the day.