Coderbyte Number Addition - Maybe I don't understand nested if? I'm lost

I cannot figure out why I am getting ['9','9']. It seems clear to me what is happening I just don't know why.

When i = 0, the last else if statement returns true, so phrase[i+1] is returned: '9'

When i = 1, somehow the second else if is returning true. Why is this? If anything, shouldn't I get ['9',undefined]? I know I'm wrong but I'm just providing my thought process.

If you have any advice or maybe the way I'm trying to do this is bad practice or anything, I would really appreciate it. I feel so dumb I've been looking at this for a while and can't seem to figure it out on my own. =(

The other glaring issue I have with my code if anyone has some thoughts is that I realize that I am only coding for the condition that the largest values I could receive are two digit numbers. I'm sure there's a much better way to do this which would allow for any numbers and negative numbers but I I'm not sure I'm at the point I can do that yet.

Thank you

function NumberAddition(str) {
    //turns string into array
    var phrase = str.split("");
    //creates variable storing empty array
    var numbers = [];

    //iterate through the array
    for (var i = 0;i<phrase.length;i++){
        //if two numbers in a row, concat then push value into new array, skip concat'd #
        if      (!(isNaN(parseInt(phrase[i]))) && !(isNaN(parseInt(phrase[i+1])))){
            numbers.push(phrase[i] + phrase[i+1]);
            i = i + 2;
        }
        //if first is a number and second is a letter, push first value
        else if (!(isNaN(parseInt(phrase[i]))) && (isNaN(parseInt(phrase[i+1])))) {
            numbers.push(phrase[i]);
        }
        //if first is not a number and second is a number, push second value
        else if (!(isNaN(parseInt(phrase[i])) && (isNaN(parseInt(phrase[i+1]))))) {
            numbers.push(phrase[i+1]);
        }  
    }
    //at this point, you should have all numbers in the numbers array
    //fyi can add strings and get value with + infront as operator
         console.log(numbers);
}
   NumberAddition("f9");