Before revision:  A greedy policy, which attempts to match agents upon arrival, does not account for the positive externality  waiting agents generate as they make it easier to match agents that arrive in the future , and may result in more unmatched than other policies.
After revision:  A greedy policy, which attempts to match agents upon arrival, does not account for the positive externality that waiting agents generate as they make it easier to match agents that arrive in the future , and may result in more unmatched than other policies.