Hello Phil A,
This is a default behavior to keep performance and avoid unnecessary traffic over the storage. I can't confirm for Mac (maybe posting in a Mac community would help) but Windows Server will always apply the "economy" of do not move all file data, blocks or chunks within the same storage.
If you need for any reason to accomplish that, I would recommend XCOPY or ROBOCOPY to move the files (first will copy, then delete source).
Hope this helps in your case,
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