Because anything you delete isn’t actually wiped from your device, whether it’s a file on your computer or a photo on your phone. It’s marked as deleted on the operating system level, and opened up as free space for new data to take its place. Somewhere down the line, something’s been messed up in the Photos app, causing this.
If you’ve ever used Disk Utility on a Mac and seen the faster vs. more secure option when erasing a disk, that’s what this is referring to. It’s faster to just mark everything as deleted on a disk, as opposed to putting several cycles of garbage data to cover up old files. And, I seriously doubt the Reddit rumor is true because I believe iOS creates and throws away an encryption key for everything on a device before it’s erased, so a new user wouldn’t be able to just stumble upon old photos.