This is not a new finding, though the linked graph is novel. The US is an obvious outlier. Iran used to be an outlier too -- more religious than expected. I can't find it on the chart, but I believe Iran is much poorer than it used to be, and perhaps less religious too.
The usual assumption is that as a nation becomes wealthy, and better educated, it becomes less religious. Of course it could be the other way around. It might be that religiosity makes a nation poorer.
That would explain Iran. And the US too, I suppose.
Most likely, however, both wealth and religiosity are more directly related to national education levels.