It is a feature. They have an algorith that determines what background needs to be darkened. And it is 100% constant. Every background you set will always have the same result when you set it as a background for the second time.
Generally it darkens light backgrounds more and darker backgrounds less and when you resize the background and move it around the opacidy of the effect changes.
But even still it is 100% constant.
I guess their idea behind it was to make the app icon texts more easily readable against white backgrounds but i think it is a very bad feature that doesn't do any good because i would rather have the freedom to set whatever background i want to use and make the decision myself to change to a darker background if i can't see the text on the app icons.
And i think it is silly to ruin everybody's backgrounds because some user has complained he can't see a text of an app.. (Just like he needs the text to find he's/her's facebook icon.
I think it only does harm. I don't see the "bright" side of it.