Sign of the surface differential rotation, α_obs, for two-spot simulations with stellar inclination angles of i = 30° (left), i = 50° (middle), and i = 70° (right). The spot latitudes (L₁ and L₂) range from −85° to 85° with steps of 5°. The yellow triangles represent the cases where only one rotation period is detected according to the criteria (see text). The red dots represent the cases where the wrong sign of differential rotation (α_obs < 0) is found, while the green squares mark the cases where the correct sign (α_obs > 0) is recovered. The dashed line divides the regions where the correct (above) or wrong (below) sign of α_obs is expected from the results of Sect. 4.1 (see text for details).