Confusing z-index

[bilogic]

noUiSlider/src/nouislider.ts

Lines 2705 to 2708 in 5703367

	// Handles before the slider middle are stacked later = higher,
	// Handles after the middle later is lower
	// [[7] [8] .......... | .......... [5] [4]
	function setZindex(): void {

What problem does this solve? Why middle?