If the dew point is air resistance is slightly lower, but it will affect the 100m a little bit and the mile not much at all (wind effects go with the square of the speed.
One possibility is that sometimes the air is calm, so wind effects are diminished.
[As for the reason for the lower resistance, think of your chemistry. Air is almost all N2 and O2 with a little CO2. Water vapor is H20. Molecular weights are 28, 32, and 44; vs 18. So, the more moisture there is for a given air pressure the lighter the air is and the lower the 'wind resistance'. For distance running, the reduced cooling quickly outweighs the minor advantage unless it is cool but if it is cool the dew point will be low and it is the dew point that matter for the air density, not the relative humidity; i.e., it is the the absolute amount of water vapor, not the amount compared to the amount air could hold at that temperture.]