sum在switch的作用域外面也可以输出，为什么会输出末尾的sum?

package com.company;

import java.util.Scanner;
public class Star_1 {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);

System.out.println("请输入年月日");
int year = input.nextInt();
int month = input.nextInt();
int day = input.nextInt();
int sum = 0;
int total = 0;

    switch(month) {
        case 12:
            sum += 30;
        case 11:
            sum += 31;
        case 10:
            sum += 30;
        case 9:
            sum += 31;
        case 8:
            sum += 31;
        case 7:
            sum += 30;
        case 6:
            sum += 31;
        case 5:
            sum += 30;
        case 4:
            sum += 31;
        case 3:
            sum += 30;
        case 2:
            sum += 31;

        default:
            sum = sum + day;

    }
    System.out.println(sum);

    }

    }

正因为在switch之外才正确啊，switch代码结束后，接着执行这个输出语句啊