Hello Mr. Gregory,

One line answer to your query would be that Kurtosis effect is dominated by the outliers, think of the distributions with same standard deviation but distinct fatter tails/peaks. Also, Kurtosis is captured/netted by the 4th order moment:

1st order moment — Mean

2nd order moment — Variance

3rd order moment — Skewness

4th order moment — Kurtosis

When you will explore the mathematical expression for Kurtosis,you will realise that it just depends on the tails and the peak of the distribution(*dominated by the former).