On defining a mirror site.....

I recently applied to become an editor and was turned down as one of the websites I recommended was deemed to be a mirror site.

True enough, the website belongs to a company which is a subsidiary of a larger company, and states this on the front page, however the two websites are completely different, and the one I recommended has a unique high quality content. Hence my recommendation.

How can this be considered a mirror site?

Hard to tell without knowing the site(s) in question.