There's an explanation here: https://www.youtube.com/watch?v=qRFz8merXEA. Briefly, the data is from weather stations, which are only interested in short-term predictions. Hence they didn't bother to recalibrate when they upgraded their instruments. The raw data should not be used for long-range climate predictions. It needs to be adjusted; in effect, to calibrate it retrospectively. There's no deception or fraud going on here.