There are a lot of things in a jpg file. Of course there's the data defining the jpg image, but there is also metadata including EXIF, IPTC, and comments. There are probably variations in metadata content depending on the jpg source.
I have seen a noticeable difference in the size of a jpg file depending on the software that generated the jpg. The following graph is included in my study on multiple writes of a jpg. I started with a tif, then saved that image to a jpg, then saved it again, and again........... The study (14 pages) can be seen at
https://static.uglyhedgehog.com/upload/nt/2023/8/28/621559-compression_study_20220206.pdfNote that ImageMagick changes its file handling above a quality number of 90.
Note also that in the middle range of quality there's about a factor of 3 between a file generated by ImageMagic and one generated by FastStone. If you really need the minimum file size I recommend ImageMagick (open source, a command-line program).
The image quality is, of course, subjective. I generally use 80 for my quality but it can depend on the image. A really busy image can show more artifacts at a larger quality number than a minimalist image (I have not yet studied that). Some images look just fine at quality 60.
Since you are talking about the 'original jpg' files, are we to assume that these are files direct from the camera? Or is some other software involved?
There are a lot of things in a jpg file. Of course... (