unexpected (temporary) free space when unlinking files on a snapshot FS

[user]

Create 8 1gig files.

(check free space on disk)

Create a snapshot.

Delete all 8 1gig files.

At this point you would expect the free space on disk to match what it was
when you just checked it earlier (see above).

However, for a small amount of time (a minute or so ?)  You actually get a
fair amount of that space back - and it is reported as free in `df`.

And then it goes back down to what it was when you made the snapshot.

Why does df report (temporarily) a gain in disk space after deleting these
files (which should not result in any disk space gain)

Thank you.