The decline isn't declining. It's the difference between an 11 year period and a 7 year period.
You know what's really interesting? Look at the population shifts at the back of the publication. In some areas of the U.S., especially the northeast, the west, and the northwest, the "none's" (i.e., zero religious belief) represent 25-30% or more of the total population. So, in an area like Alabama, religious belief has declined, but not as drastically as it as in places like Wyoming, Nevada, Montana, Delaware, New Hampshire, and Maine. Those places are outright heathen.
If you live in an area with heavy religious influence, you probably believe that religion is remaining strong. But, if you live in the Northwest, northeast, atlantic states, or the west, you're living in a place with a tremendous growth in non-religious types. It's kind of a self-affirming sentiment because America is increasingly dividing itself regionally along political and religious lines (which, not oddly, fit).