From GW's perspective, each of their top charities can consume a certain amount of additional money before the expected value of an additional donation decreases by some amount. Their goal is to move money such that each charity hits that target, and then they'll reassess. So they recommend donors split donations so that, as a whole, these targets are hit and EV is maximized. From your perspective, you may decide that concentrating your entire donation in one organization has a higher EV, since that organization has a generally higher EV relative to the others and since your action isn't going to affect the actions of the rest of GW's audience.