On disk vs logical size
Why two numbers, and which one to trust.
The two measurements
On disk is the space the filesystem has actually allocated. Logical size is the length the file reports. DiskPeek shows on disk first, because that is the number that matters when you are trying to free space up.
Why they disagree
Sparse files claim more length than they occupy. Compressed files occupy less than they claim. Clones share their blocks with another file entirely. All of these make the two numbers diverge, sometimes wildly.
Why deleting 4 GB might not give back 4 GB
On APFS, clones, snapshots and shared blocks mean the space a file appears to hold may also be held by something else, so removing it frees less than its size suggests. Time Machine local snapshots are the usual culprit.
DiskPeek says so rather than promising a number it cannot deliver. It is the honest answer, even though it is the less satisfying one.