Charity from those who believe in afterlife rewards must be greeted with some suspicion because such persons can never be entirely free of the idea that they will be rewarded for their actions, if not in life then in death. No one can be sure, least of all the believers themselves, if their actions, no matter how charitable, are in fact selfishly designed to improve their own lot in the afterlife.
The genuine atheist, however, at least has the possibility of performing an act they believe can do themselves no good at all.
To be sure, atheists are perfectly capable of selfishness. But unlike their believer brethren, they might from time to time do something genuinely altruistic.