Why do the square pixels of a figure change the shape and size after saving?

I try to save or export figures I created in Matlab but the square pixels have different shapes in the saved image. I also tried saving the figure as different imagetypes but I never got an image that has the exact same pixels as the matlab figure. In the first picture you can see a screenshot of the pixels in the matlab figure.

In the second picture you can see a screenshot of the pixels of an exported version of the same figure.

[Image Analyst]

Duplicate of http://www.mathworks.com/matlabcentral/answers/60687-why-do-the-square-pixels-of-a-figure-change-the-shape-and-size-after-saving