Why does my blu ray movie switch between widescreen and full screen between scenes?

I just bought The Dark Knight, my favorite movie, on blu ray. I put it in my ps3 and started watching it on my vizio HDTV(1080p). While watching I noticed that,even though it all was in hd, some of the scenes were full screen and some of the scenes were widescreen. Why is this? Is it my tv? Or the way they filmed the movie? Or even both? I'm just curious why it was like that.Why does my blu ray movie switch between widescreen and full screen between scenes?
I have a vizio HDTV and an insignia Blu-ray player



It's a widescreen tv



I get something I believe is called "over scan" where it's the wide screen picture but its without the black bars on top and bottom