Why does the 1st array element have index 1 in the MDN documentation?

Question

JavaScript

I read a guide for understanding reduce in JS
In general, in this article there are 2 tables, in the 1st for some reason the index of the 1st element is 1! Is this a bug or am I misunderstanding something? It is important to note that everything seems to be in order in the second table.

Answer 1

If the initialValue argument is not given, then the value of accumulator will be equal to the first value in the array, and the value of currentValue will be equal to the second value in the array.