I don't know for sure, but I am under the impression that they just do one transaction each month to each charity, rather than multiple (such as whenever someone donates).
One charity that I support used to have a link on their website just for a PayPal donation, and somehow I ended up giving to them via PayPal Giving Fund instead, and they told me that PayPal Giving Fund ended up giving them 100% of the donation amount, whereas with a standard PayPal donation, some percentage was kept as a fee.
So maybe the PayPal Giving Fund delayed schedule is related to them not charging the fee?