tar --incremental problem

I am using GnuWin32's version of tar 1.13 on Windows XP. I am having trouble using the --incremental option. Here is an example. I have made a directory Files which contains the file a.txt. I make a full backup of it using

C:\Test>tar --create --verbose --file=files.tar Files
Files/
Files/a.txt

Now I delete a.txt.

C:\Test>del Files\a.txt

Next I create an incremental backup of the contents of Files.

C:\Test>tar --create --verbose --incremental --file=files_inc.tar Files
Files/

Next I delete Files.

C:\Test>rmdir /s /q Files

I try to restore with my tar archives:

C:\Test>tar --extract --verbose --file=files.tar
Files/
Files/a.txt

C:\Test>tar --extract --verbose --incremental --file=files_inc.tar
11071337570/Files/

What is this string of numbers before /Files/? If I understood how --incremental works, I expect that the file a.txt should be deleted from Files when I extracted files_inc.tar. However,

C:\Test>dir /b /s
C:\Test\11071337570
C:\Test\Files
C:\Test\files.tar
C:\Test\files_inc.tar
C:\Test\11071337570\Files
C:\Test\Files\a.txt

A new directory has been created whose name is that string of numbers, and it contains Files. Also, the file a.txt has not been deleted.

Am I using --incremental incorrectly? What do I need to go to get the desired behavior?

Thanks.