Since no one else answered your question (the why) here is the deal:
Successfully recovering deleted files is never straightforward and depends on several factors.
- The medium: SSD or HDD
- The PC usage we do AFTER the time we erased those files
- Other reasons that have to do with how windows (or any OS) handles the file system.
When we delete a file, the file never gets wiped instantly, but rather the OS marks the area that those files occupied as ready to be used again. So if one copies or creates several files after the deletion, there is a good chance that some of those files will overwrite some or all of the files we have erased in the past. Note here that if one uses the OS disk as storage for personal documents, the chances of recovery are even smaller as the OS and the apps we use generate a LOT of temp files that will wipe erased files. In contrast, if we use a separate disk, this will increase the chance of finding deleted files, even older ones.
The above is mostly for HDDs (the spinners) because SSDs make things worse. SSDs because they use memory cells and not magnetic media to store data, they need to wipe those cells before they are able to be rewritten again, otherwise when we erase files the disk will still report the cell as used, since the OS as we said above does NOT free the area (wipe). The process of wiping cells is called TRIM (others call it active garbage collection) and it speeds the SSD disk when it comes to write speeds BUT at the cost that if a TRIM command is issued, erased files are permanently gone. Users have no power over TRIM the OS issues that automatically at set intervals.
Of course, all the above are a bit oversimplified as the real internal works for hard disk and file systems, could cause headaches.
So in your case, it could simply be that the OS for some reason overwrote the area where those 6 pictures were using, while it didn't for the others. This has nothing to do with the time you deleted the files, but rather their size and location on the disk, if the disk Is SSD or HDD etc. Note here that even if you see a successful recovery the files should still be checked manually cause often you may find corruption in them.
That doesn't mean you should stop looking, but the software won't change things by much as every software just uses the same NTFS commands to check for deleted files.
Hope it helped.