Preview JPEGs when there is no Preview JPEG

I'm trying to understand how I can extract the preview JPEG from a DNG created with the DNG Converter and be SURE I am getting the preview JPEG not a JPEG created from the raw file.

I just did some tests creating DNGs using the DNG converter (v16) where I generated some DNGs with Large JPEG preview embedded and some with no preview. I do see the the DNGs without a preview JPEG are smaller, so it does seem like there no preview JPEG in them.

However, when I use EXIFTOOL to extract the preview image using "exiftool -b -PreviewImage -w .jpg" I DO get JPEGs for SOME of the DNGs that did not have a preview image embedded in them - which I did not expect to see.

Specifically, Compressed DNGs with no preview JPEG did result in exiftool outputting a preview image, while Uncompressed DNGs did not. The JPEG that was output from the Compressed DNGs is very small (8kb vs 50kb for those with preview images).

Can anyone explain why this would be the case? I just want to extract the preview JPEG and get nothing back if the DNG doesn't have one...