You have to remember, these are all studies where people identify their own religion. There is a whole world of psychological pressures that affect one's answer. People don't always respond truthfully (whether consciously or subconsciously) under those pressures. Humans naturally adapt their self-image to fit situations.
Heck, I remember when I was just starting to talk, my parents told me I was Christian and I believed them. If you had asked me if I was Christian, I would have said "yes". I didn't even know who Jesus was or a single moral pillar of the religion. Sadly, there are people whose dedication in adulthood is no deeper than mine as a child.
The reason I'm bringing all this up, is that I don't think the US is as 'hardcore Christian' as that graph would suggest. There are a lot of people who identify as Christians who don't go to church, don't follow any Christian moral guidelines, and support separation of church and state.