1953 Bowman Color Question

mudflap02

I've been meaning to ask this for a while. These cards look absolutely great, but I just don't get something about them. Why is it that virtually every card i bright towards the middle, then darker towards the corners? It's like youre looking at the card through a pinhole. MAybe there's no reason for this, but it's just always struck me about the set. Some card like the Reese and Musial just stand out in my mind as examples of this. Any experts out there in the peanut gallery want to set the record straight?