To be fair, they only left out one actual data point from their average. The rest of the countries show n.a. for the data point where the average is calculated from.
Yet because there are only 7 data points possible, it completely changes the result. This is a very high profile economic report coming from two very influential Harvard economists. Consider that they accidentally left out their -7.9 data point from New Zealand:
Not only does removing the NZ data point instead make the average much higher, the data looks more reliable than the full dataset as the standard deviation has almost been cut in half. Leaving out one data point from such a small set has big consequences. Their 6 data points: ave: -0.07, s.d.: 3.83
All 7 data points: ave: 0.26, s.d.: 3.67
No NZ data point: ave: 1.43, s.d.: 1.73