With this illusion, it causes your brain to interpret the colors as it thinks they should be instead of what they really are. Since Block A is next to lighter shaded squares, and Block B is next to darker shaded squares, It makes it seem like squares A is darker than it really is. The illusion which is called "The checker shadow illusion" shows us that everything isn't as it seems.
"Adelson's Same Color Illusion." Brain Exercises, Brain Fitness, Brain Training. N.p., n.d. Web. 18 Mar. 2015.
"Related Materials." Edward Adelson's Checker Shadow Optical Illusion. N.p., n.d. Web. 18 Mar. 2015.