I don't like generalizations such as these. With such issues it is important to define religious from spiritual. The term "Religious" is loaded. Athiests can be spiritual not religious, be very careful with putting stigmas on groups of people, hitler did that, its a nono.
I'll be Bill clinton in a good way and ask "what do you define charity as". Charity to some may be giving it away for taxes. Charity to some may be giving it to an entreprenuer in Ukraine, africa, or brazil for a very small interest percentage as compared to local interest rates to give some pride to those receiving the funds who now have the incentive to work to pay it off and make more money. Be very careful with your postulates, and define your terms clearly.
"Athiests" tend to be younger than "religious" people, who have grown and learned the ropes. Young people make less money= less donations.