Why does this mergeSort implementation use mergeSort(arr, low, mid) and mergeSort(arr, mid, high) instead of mid+1 in the second recursive call?

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 + " ");
    }
}