In the provided mergeSort implementation, the recursive calls are mergeSort(arr, low, mid) and mergeSort(arr, mid, high). Why is mid used as the starting index for the second recursive call instead of mid + 1?

public class MergeSortDebug {
    public static void mergeSort(int[] arr, int low, int high) {
        if(low < high) {
            int mid = (low + high) / 2;
            mergeSort(arr, low, mid);
            mergeSort(arr, mid, high);
            merge(arr, low, mid, high);
        }
    }
    
    public static void merge(int[] arr, int low, int mid, int high) {
        int n1 = mid - low;
        int n2 = high - mid;
        int[] left = new int[n1];
        int[] right = new int[n2];
        for(int i = 0; i < n1; i++)
            left[i] = arr[low + i];
        for(int j = 0; j < n2; j++)
            right[j] = arr[mid + j];
        int i = 0, j = 0, k = low;
        while(i < n1 && j < n2) {
            if(left[i] <= right[j]) {
                arr[k++] = left[i++];
            } else {
                arr[k++] = right[j++];
            }
        }
        while(i < n1)
            arr[k++] = left[i++];
        while(j < n2)
            arr[k++] = right[j++];
    }
    
    public static void main(String[] args) {
        int[] data = {4, 2, 7, 1};
        mergeSort(data, 0, data.length);
        for(int num : data)
            System.out.print(num + " ");
    }
}