In the calculation of the Human Development Index, Gross National Income (GNI) is used rather than GNP.

I've searched around on the internet, but I haven't found a clear explanation of what the precise difference between GNI and GNP is. Wikipedia defines GNI as "total domestic and foreign output claimed by residents of a country" and the GNP as "market value of all the products and services produced in one year by labour and property supplied by the citizens of a country." Just like the income and expenditure approaches to calculating GDP yielding the same result, it seems to me that GNI and GNP should be at least theoretically equal.

What is the difference between GNP and GNI, and why use one over the other for measuring development?