Explain why the air on a hot humid day is less dense than on a hot dry day. The dew-point is the temperature that the air must be for condensation or dew to form. If the two numbers are close then the air must near saturation. Since condensation is a heating process, as the temperature of a wet bulb thermometer reaches its lowest point possible, it is releasing heat and raising the temperature slightly. Thus the wet bulb temperature is slightly higher than the dew point.
Absolute humidity is a mass to volume ratio and a variety of factors can influence the volume.
If altitude increases, the pressure decreases and the volume as a result, decreases. Think of clouds as a reminder. Actual vapor pressure decreases slightly due to the reduced pressure inside the rising, expanding parcel. Saturation vapor pressure decreases as the air cools because of the lower air temperature.