The percentages aren't of total population, but of the individual categories under consideration..
"Forty percent of worship-attending Americans volunteer regularly to help the poor and elderly, compared with 15% of Americans who never attend services."
I don't understand what you're getting at with the proportional thing. 40% of us..., compared to 15% of you...
And how does DWB factor in? We give to secular organizations, too. As mentioned in the article.