Why is the last array pushing 0 as the largest number in the last array?

```
function largestOfFour(arr) {
let result = [];
for (let x = 0; x < arr.length; x++) {
let trackNum = 0;
let subA = arr[x];
for (let y = 0; y < arr[x].length; y++) {
let num = subA[y];
if (num > trackNum) trackNum = num;
}
result.push(trackNum);
}
return result;
}
console.log(largestOfFour([[17, 23, 25, 12], [25, 7, 34, 48], [4, -10, 18, 21], [-72, -3, -17, -10]]));
```