Why do static files get defragged again?

Defraggler
1

My experience of defraggers is that once a file is defragged that unless a new copy is written to disk or otherwise the file is updated that that same file should never be defragged again, nor need defragging - ever.

With defraggler I see files reappearing on the defrag list. A couple these are huge files - and AVI and an ISO file, so we are talking 100s megs. Another one is an Ubuntu disk image, which is fixed at 8gigs.

With large files such as these re-dragging can be painful, even on my fast disk.

2

Are those files - by any chance - sparse or compressed? Because then they would fragment whenever they are written to, even when their size remains the same. Other than that, I don't know how such files could become fragmented...