Why does the volume of the speakers drop after taking out the headphones?

Question

macOS

Initially, the volume is normal, then I insert the headphones, after pulling out the volume of the speakers drops by about half.
macbook pro 15 2015
Mac os Mojave 10.14
PS This means the maximum volume drops (I.e. at maximum volume, the sound is quieter than before turning the headphones on and off)

Answer 1

Experiment:
0. Turn on the music
1. Unplug the headphones and set the volume to 10%
2. Plug in the headphones and set the volume to 100%
3. Unplug the headphones and listen. The volume should become equal to 10%
4. Plug it back and the volume will be more than 10% and less than 100%. For many devices prohibit by default the maximum volume for headphones, so that you do not accidentally go deaf.
With such a simple experiment, a typical PC user can confirm or refute the separate existence of the system and volume of external media devices.
In Android, just do everything separately: system (speakers), wired and wireless headphones.