It is simple leverage you learned in physics. 😉
The track pad is hinged at the top, intending that you will press to click at the bottom. The further from the hinge line you press, the more leverage you will have to effect the click. So, conversely, the closer you press to the hinge line, the more pressure will be required to operate the switch.
I also like tap ... so rarely click the trackpad other than to login.🙂
-howard