Because you're placing a higher weight on elements that come later in the list. Try it for the list [1, 2, 3, 4] The mean is 2.5 if you sum them all and divide

Because you're placing a higher weight on elements that come later in the list. Try it for the list [1, 2, 3, 4] The mean is 2.5 if you sum them all and divide by 4. With that method it is: mean([1, 2, 3, 4]) = mean([1.5, 3, 4]) = mean([2.25, 4]) = 3.125