Why does tar command create this directory structure when extracting a tar file?

I have created this simple directory structure:

$ tree testdir/
testdir/
├── subdir1
│   ├── file1.txt
│   └── file2.txt
├── subdir2
│   ├── file3.txt
│   └── file4.txt

and would like to archive the contents of testdir into a tar file. So I run the following command (note the exclude is because of this answer ):

tar cvf /home/myuser/testdir/testdir_tarfile.tar --exclude=/home/myuser/testdir/testdir_tarfile.tar /home/myuser/testdir

Now I want to extract the files (along with their directory structure) once again. So first create a target directory:

$ mkdir /home/myuser/testdir/target_dir

and now extract the tar file to the target directory:

tar xf /home/myuser/testdir/testdir_tarfile.tar -C /home/myuser/testdir/target_dir/

The resulting file structure looks like this:

$ tree testdir/
testdir/
├── subdir1
│   ├── file1.txt
│   └── file2.txt
├── subdir2
│   ├── file3.txt
│   └── file4.txt
├── target_dir
│   └── home
│       └── myuser
│           └── testdir
│               ├── subdir1
│               │   ├── file1.txt
│               │   └── file2.txt
│               └── subdir2
│                   ├── file3.txt
│                   └── file4.txt
└── testdir_tarfile.tar

Why does the target directory now contain subfolders "home" and "myuser"? I would like it to just contain "subdir1" and "subdir2", i.e. like the original archived directory.