I thought the same so looked for the original article on Google Scholar. From Fehr and Gachter, "Altruistic Punishment in Humans":
... groups with four members played the following public
goods game. Each member received an endowment of 20 money
units (MUs) and each one could contribute between 0 and 20 MUs to a group project. Subjects could keep the money that they did not contribute to the project. For every MU invested in the project, each of the four group members, that is, also those who invested little or nothing, earned 0.4 MUs. Thus, the investor's return from investing one additional MU in the project was 0.4 MUs, whereas the group
return was 1.6 MUs.
Your return on an investment of 1U is 0.4U, so it's always best for you personally to keep a unit. But the group return is 1.6U so the group as a whole is enriched for every unit invested.
... groups with four members played the following public goods game. Each member received an endowment of 20 money units (MUs) and each one could contribute between 0 and 20 MUs to a group project. Subjects could keep the money that they did not contribute to the project. For every MU invested in the project, each of the four group members, that is, also those who invested little or nothing, earned 0.4 MUs. Thus, the investor's return from investing one additional MU in the project was 0.4 MUs, whereas the group return was 1.6 MUs.
Your return on an investment of 1U is 0.4U, so it's always best for you personally to keep a unit. But the group return is 1.6U so the group as a whole is enriched for every unit invested.