More accurately, the index says you have more space since that file no longer accounted for.

It's like assuming the card catalog at the library is always 100% accurate and correct. Until you come across a book on the shelf without a tag. The book is there but the card catalog doesn't have it cataloged. It's much more complicated than that, but that's the jist of it.

Some burners are able to ignore the write once info from the cd/dvd and continue to write to it.