FSCTL_SET_ZERO_DATA leaves 16 clusters allocated

NTFSD
1

I’d like to use sparse files to deallocate the files from the disk and
maintain their size. After FSCTL_SET_ZERO_DATA is issued on whole file, my
experiments show that 16 clusters of space is left allocated. This is
typically 64kb. I’d like to decrease the amount of allocated size, if
possible to zero.

Do you know of any possibilities to achieve it?

Joze