I shall try to give a non-mathematical explanation.

Do you see that in each monththe the average for each area considered in isolation increased?

Do you also see that in each month the average for area 1 was lower than the average for area 2?

But the difference in hours worked between June and July was relatively modest in area 1 but relatively large in area 2?

So the average monthly change for the whole company is determined by the difference in the average monthly wages paid in the two areas **AND** by the month;y difference in the hours worked in the two areas. In this specific case, the greater reduction in the hours worked in the more costly area more than offset the increase in wages in both areas. In other words, more than one thing is going on, and you cannot focus on just one.

Does that make intuitive sense?

There is a way to eliminate the problem, and that is to take both into account, but the math can get a bit tricky to explain.