This is strange to me because my understanding is that era5 is reanalysis data based on models and data assimilation (i.e. bayesian time series modeling). So they produced a machine learning model that predicted modeled data but not the raw data. But I could be completely wrong as Im not an expert on this.

link for era5 https://www.ecmwf.int/en/forecasts/datasets/reanalysis-datas...