They thought it was unexpected because it was unexpected. From the abstract: "These findings stand in stark contrast to the (incentivized) predictions of both

They thought it was unexpected because it was unexpected. From the abstract: "These findings stand in stark contrast to the (incentivized) predictions of both experts and a nationally representative sample of laypeople, who - depending on the treatment group, outcome, and time period - estimated treatment effect sizes of +0.16 to +0.65 SDs."