Copying vob file - md5sum does not match?

Stated with a DVD, unencrypted, and all region (no region encoding).

I wanted to copy the VOBs to my harddrive, individually.

I first used Toast (on the Mac) to copy the one .vob file I was
interested in, and burnt this on a DVD+RW. I also had Toast run the
"verify" step, and it said the files are matching.

When I did a "md5" on the original DVD, and the copied DVD-RW, the md5
sum's don't match. And if I do a "cmp" command line command, the files
are different!

But the DVD+RW.vob file plays correctly, and as far as I can tell,
looks like the same as the original DVD.

I then tried another experiment - I started "Terminal" (shell) on the
Mac, and used FTP to transfer the file from the DVD onto the harddrive.
Did a md5 sum on this file on the harddrive, and it matches the md5sum
on the original DVD.

Can anyone explain what is going on here? Are there some bits on the
original DVD that get changed when doing a DVD to DVD+RW copy?
If yes, are those bits of any importance?