difference in actual ruler and indesign's inches scale.

I am pretty new to indesign. I have set to measure i inches. Today I had an actual physical-ruler(a regular one that is by students) lying around so I picked it up and compared it with the scale on screen in indesign. My ruler looks somewhat like the image below.

To my surprise, I found a difference. Note that there are 16 points/stops that makes an inch. On comparison, I found the on-screen-scale to be one point shorter than an actual physical-ruler. I assume it is not supposed to be like this, what am I missing ?