Gavin made up a fake 4-8 degree April anomaly across a region of Siberia and the Arctic where he had no data. Note the green circle.
Now compare vs. the green circle in satellite data, which was actually measured. Gavin missed by 4-8 degrees in that region, and thus was able to declare that April was 2nd hottest ever.
It doesn’t take a very large area of fake +8 anomaly to bump global temperatures up by 0.3C.