I too have noticed the same symptom. My theory is the amount of, and/or type of, gaskets used around the frame of the doors, as well as the opening of the doors. There is quite a bit of rubber and felt material used to help keep down the wind noise. It is this material that causes some of us to use a little extra effort when closing our Regal's doors.
By the way, I was recently at an auto show and noticed that higher end automobiles use felt around the door frames, as well. I had never really noticed this until purchasing the Regal.