When you look at the index, you're looking at the position of the values, rather than the elements, which are the values withing a given index.

So when you display the element of an odd index, you look for position/index of an array which is odd, and you display the value inside that array position/index.

On the other hand, to display every even element, you have to look at every index and check if the value is even, and if so, display it.