My guess is that newer units remove considerably more humidity than those made a couple of generations ago. The process of converting water vapor to a liquid state releases heat. This heat energy as well as the heat generated due to inefficiencies in the unit itself are released into the room.
Water vapor will only condense onto another surface when that surface is cooler than the dew point temperature, or when the water vapor equilibrium in air has been exceeded. When water vapor condenses onto a surface, a net warming occurs on that surface. The water molecule brings heat energy with it.
»en.wikipedia.org/wiki/Water_vapo ··· er_vapor
The enthalpy of condensation (or heat of condensation) is by definition equal to the enthalpy of vaporization with the opposite sign: enthalpy changes of vaporization are always positive (heat is absorbed by the substance), whereas enthalpy changes of condensation are always negative (heat is released by the substance).
»en.wikipedia.org/wiki/Enthalpy_o ··· rization
For even more exciting reading......
»en.wikipedia.org/wiki/Latent_hea ··· ent_heat--