问为什么无论我是否运行git add，当我切换分支(修改、添加、删除文件)时，git都会一直显示我的更改？

EN

$ git clone <a_repository>  
$ git branch  
* master  
$ git branch testing  
$ git checkout testing  
...edit a file, add a new one, delete...  
$ git status  
    # On branch testing  
    # Changed but not updated:  
    #   (use "git add/rm <file>..." to update what will be committed)  
    #   (use "git checkout -- <file>..." to discard changes in working directory)  
    #  
    #       deleted:    file1.txt  
    #  
    # Untracked files:  
    #   (use "git add <file>..." to include in what will be committed)  
    #  
    #       file2.txt  
no changes added to commit (use "git add" and/or "git commit -a")  
$ git branch  
  master  
* testing  
$ git checkout master  
D       file1.txt  
Switched to branch 'master'  
$ git status  
    # On branch master  
    # Changed but not updated:  
    #   (use "git add/rm <file>..." to update what will be committed)  
    #   (use "git checkout -- <file>..." to discard changes in working directory)  
    #  
    #       deleted:    file1.txt  
    #  
    # Untracked files:  
    #   (use "git add <file>..." to include in what will be committed)  
    #  
    #       file2.txt  
no changes added to commit (use "git add" and/or "git commit -a")