I wouldn't be sure that the numbers are correct.
Here in the UK 59% of the population claimed to be Christian in the 2011 census. However attendance at churches of all varieties runs to about 6% of the population. So what happened to the other 53%, are they really Christian or merely putting themselves down as Christian because it sounds better?
One thing that has been reported in the past is that while 40% of the population of the States reports that they attend services each week. However when actual counts are taken the figure is only about 20%
Conclusion? The 77% might be overselling the number of people who are actually Christian compared to being cultural Christians.