I think, it has to do chest voice vs. head voice.

Basically, when you vibrate your vocal cords, then depending on the frequency, it can either resonate with your chest cavity or with your head cavity. The chest cavity is larger, so it resonates at a deeper frequency and the resonance is louder, which is why most of us use chest voice while talking normally.

But yeah, as you go up, there's a bit of a range where you have to put more force into making your vocal cord vibrations heard, because at those frequencies, it does not resonate well with either chest or head cavity.

And then beyond that, you get into the range where it resonates well with the head cavity, so it's again not as taxing anymore to speak in that range (although still usually more taxing than chest voice, because it just resonates less loudly).

So, even with my deep chest choice, I do also have relatively good range into the upper registers, because well, my head cavity isn't particularly larger than others'.
But I have heard that some people cannot tap into their head voice, not without vocal training anyways.