Why is My Voice Deep As A Girl?

These could also apply to men with higher-pitched voices; just think the exact opposite of the examples I give.

How?
The sound of your voice is determined by your larynx; the shape or size decides the vibration and sound that it produces. The structure of your larynx is defined by genetics and hormones.

Why?
Longer or wider larynx tend to have a deeper sound come out because they vibrate much slower. An example of this is when you use a thicker string on a guitar it will play a deeper note. Your vocal cords could also have muscles that stretch them, producing a lower pitch.