The variance of the first 7 natural numbers 1,2,…,7 is

For the first $n$ natural numbers the variance is $\dfrac{n^2-1}{12}$. With $n=7$: $\dfrac{49-1}{12}=4$. Directly, the mean is $4$ and the squared deviations are $9,4,1,0,1,4,9$ with sum $28$, so the variance is $\dfrac{28}{7}=4$. The value $2$ is the standard deviation, which is a different measure, and $16$ is the square of the mean.