Sorry, vertical axis "increase" should be decrease
It is because of the viscosity of fluid. Air is fluid. Sound source located low, on ground, propagates in air bending up from high to low viscosity direction. Sound source located high, in air, will bend downwards. (Similar to the refraction of light.)
Air flows from low position to high because of the temperature difference. Temperature for air near the the earth is usually higher. Hot air will flow upwards and voice transfer faster upwards.