Possibly EAs think about their donating more and are thus more likely to be able to look up the exact sums they've donated. While non-EAs might be more likely to just donate to some random thing that seems like a nice enough cause, and then forget the details.
If EAs are reporting actual numbers they've looked up and non-EAs are reporting their vague estimates of how much they might have given, I would expect non-EAs' figures to be worse overestimates than EAs'.
http://www.gwern.net/EA%20donations