eyedropper says the bottom one is lighter! I thought they where the same..

still top piece is darker

Yeaa, I agree, top one still darker to me.

Image: Which one is darker?
Me: Hmm.. The top one looks a bit darker.
Image: Now put your finger or a pen between them and look again.
Me: Jup.. Top one is darker.

How can you call this an illusion? Or is it the illusion that this is an optical illusion?

put something larger than pen and they have the same color

but technically they don't because the bit you cover up is part of the block, so the top one is darker, unless you change the image, so thats not an optical illusion

on the bottom one only the top part is lighter to push the optical illusion just save it open it in paint highlight large portion of the center of both top and bottom and swithch the cut out part and it is same color.

use photoshop then click EYEDROPPER TOOLS
they have the same color

I told my kids we'd play after I found what I nedeed. Damnit.