请帮忙看看这段程序怎么理解！

#include<iostream.h>
 void main()
 {
     int score,s,sum;
     char grade;
     cout<<"please input score "<<endl;
     cin>>score;
     if(score>=0&&score<=100)
     {
         s=score/10;
         switch(s)
         {
             case 0:
             case 1:
             case 2:
             case 3:
             case 4:
             case 5: grade='2';break;
             case 6:
             case 7: grade='3';break;
             case 8: grade='4';break;
             case 9:
             case 10: grade='5';break;
         }
         cout<<"grade is "<<grade<<endl;
     }
     else
         cout<<"data error\n";
 }
这是一个把百分制成绩按照五分制输出的程序，用switch写的！可是我不太明白case那段代码。比如我输入20，为什么也显示2，输入30也显示2！请知道的解释下具体原因！