Because the pixels are not square. You chose to use an HDV setting which has a Pixel Aspect Ratio of 1.333 - check the video section of the picture you posted.
There are only 1440 pixels but they are wider than they are tall, so it only takes 1440 of them to take up the same space as 1920 square pixels. (Let it be all you gurus out there - a more detailed and more accurate explanation might be required, but probably not.)
Why? That is a very long story. If you are not capturing or using HDV, you will want to select a different preset. One with square pixels (Pixel Aspect Ratio of 1.0)
HD and HDV are not the same beast!
Both use a frame whose vertical dimension is 1080 pixels. The newer "full" HD format uses 1920 square pixels horizontally to give the correct 16x9 frame. The older HDV format uses 1440 rectangular pixels to achieve the same 16x9, as Steven says.
My guess is your Nikon records the 1920x1080 format, so you need to select an HD preset..
The easy way to generate a correct sequence is the drag a clip onto the "Create New Item" icon ant the bottom right of the Project panel
Alan has made a very good point.
The more I get used to using CS6 (I skipped CS4 and CS5) the more I realize that the initial preset is almost irrelevant other than the part where you decide to capture DV or HDV from tape based cameras.
I used to create a new project every time I wanted to try out something I read about on the forum, or in preparation to answer a question.
Lately, I seem to just open "Untitled1.prproj" over and over again, adding in footage and creating new sequences from that footage. Then I don't save it when I am done. I don't care what the project was originally since it does what I want it to do with any footage I feed it.
However, for many people, I would still suggest picking the right preset, just to avoid confusion.
Steven L. Gotz wrote:
I have got the same bad habit.
When I make a new scene / projetct etc I just take whats there and later I never save anything but rhe final video.
Oh, don't get me wrong. I save, and save often, and save as, and everything I can do to make sure a project and all of my work is protected when it is one of my own projects. But I often open Premiere Pro just to make sure I am answering a question on the forum correctly. Those I don't save. I just use the same one over and over again.