3 and 4) are still from the third Thursday of the previous month. 1) has just been updated, while the model-based forecasts (Figs. The charts on this Quick Look page are updated at two different times of the month, so that between the second and the third Thursday of the month, the official forecast (Fig.
Another reason is related to the fact that the models are not run at the same time that the forecasters make their assessment, so that the starting ENSO conditions may be slightly different between the two times. One possible reason is that the human forecasters, using their experience and judgment, may disagree to some degree with the models, which may have known biases. There can be several reasons for differences.
However, occasionally they may differ noticeably. Figures 1 and 3 (the official CPC ENSO probability forecast and the objective model-based IRI ENSO probability forecast, respectively) are often quite similar.