So even if it goes above 30 every single remaining year it will still be 9 vs 13 in the previous decade?

These are 20 year periods, not decades. How did you think there were 13 years in the previous decade? It would be 18 vs 13 if every year up to 2039 went over 30C.

They’ve rendered that in a rather confusing way. 13 incidents is for _two_ decades, not one.

The time frames they are looking at are 2 decade intervals, not 1 decade