On VHF and higher frequencies, why does the radio horizon extend beyond the visible horizon?

Radio waves at VHF and higher frequencies are slightly bent (refracted) as they travel through the Earth's lower atmosphere, known as the troposphere. This bending occurs due to gradual changes in air density, temperature, and humidity as altitude increases. Because of this normal tropospheric refraction, the radio horizon extends roughly 15% farther than the true visual horizon. The D-region is part of the ionosphere and mainly absorbs lower frequencies, while soil conductivity is a factor in HF ground-wave propagation, neither of which explain the VHF horizon extension.