bottapress on "[Plugin: Autoptimize] body background color changed to grey"

I built two strictly identical test sites in order to try to understand what happens.

The 1st one, with just the CSS optimization activated on Autoptimize:

http://autoptimize.wordpress-hebergement.fr

As you can see, the body background color is #e6e6e6 (go to the bottom of the page to see the background color)

The second one, just the same without any optimization activated on Autoptimize:

http://without-autoptimize.wordpress-hebergement.fr

And here, the background color is white.

Do you find something in it that could explain the trouble ?