Why do all magnetic field of a bar magnet do not start and end only at pole? Iam very confused that why not all field lines emanate out of a pole? Why few field lines emanate and in middle of a bar magnet as shown in the following figure: 

 A: The magnetic field of a bar magnet is made up from the individual magnetic dipoles of the outermost electrons in the iron atoms. The magnetic field of an individual electron is tiny, but since there are gazillions of electrons in a typical bar magnet their fields combine to generate a substantial overall field.
If you looked at the field from a single electron it would look a bit like:

(image from Wikipedia)
so the field lines do splay out sideways as well as forwards and backwards. However in a collection of electrons the field from any given electron interacts with the electrons in front of it and behind it, and the result is to channel the field lines:

I fear my drawing skils don't do the subject justice, but hopefully the general idea is clear. When you have a bar magnet stuffed with electrons that are all lined up with each other then the field lines tend to run along the lines of the electrons so the only emerge at the ends of the bar magnet where the lines of electrons stop.
A: Why should they? :)
If the magnet is infinitely long, you will have no field lines escaping through the mantle, simply out of symmetry considerations. 
But for a magnet of finite size the field will be less strong at the ends than in the center. This sounds logical, doesn't it. And it implies, that the density of field lines has to be less in the ends, which means that they have to have escaped in between. 
