this is not an explanation for this-perceptibly darker to the edges
this effect is fundamental to the nature of projection coupled with reflection of light off diffuse surfaces-
projection surfaces dont come close to ever giving a perfect diffuse reflection of a surface- with even reflected light in all directions-
this is a very complex subject - and nothing to do with projectors themselves and all about material properties.
road signs are a red herring - they would make a terrible projection surface- - why dont you try one?
then try a piece of white laser printer paper?
which one makes the best surface?
http://en.wikipedia.org/wiki/Diffuse_reflection
http://en.wikipedia.org/wiki/Specular_reflection