Stretched or squished screencaps?

[MoviePropMaster2008]

I am not picking on any one IMFDB member (and if I posted your pic, it was random) and I've seen several IMFDB members doing this.

I am confused as to why some people's screenshots are 'squished' i.e. anamorphic distortion from widescreen to full screen.

Example:

This was posed on a page when it should look like this.


I know some folks have little control over their screencapturing software, but why is this occurring?