I think it's because the numbers aren't correct. If you climb at 13 MPH and descend at 23 MPH, you won't be averaging 18 MPH. Also note that your descent speed today dropped to 21 MPH from the normal 23 MPH.
That said, a wind's effect on speed is different uphill versus downhill. Consider the following extreme. On a 10% grade, a 5 MPH headwind won't slow my sustainable speed down significantly since aerodynamics at my pathetic speed is inconsequential and virtually all the power is going into fighting gravity. (So the aerodynamics is as if I'm going through still air at 10 MPH. Big deal.) A 5 MPH tailwind could conceivably increase my descent speed by about 5 MPH since I'm coasting downhill and limited primarily by aerodynamics.
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