Why are my photos showing up on other family member's devices?

I was looking through the photos app on my son's iPad, and I see that all of my photos - which are NOT in a shared folder - are showing up on his (and my other son's, and my wife's) device. I thought that only SHARED FOLDERS could be viewed by all family members on my shared plan. However, none of their photos show up in my Photos app.

Note: to fill this out, I'm required to select a device and topic, but this issue is not device-specific. It's related to my family's shared plan and is happening on the Photos app on all of devices, OSX and iOS.