Is my guest network set up wrong? Or do I just misunderstand it?

I was recently playing around with the mobile app fing. I had an old cell phone on my guest network and scanned it.
I was surprised to see a listing of all devices on my main network as well as guest network devices. How can that be? I would have thought that the guest network would be separate and unable to see devices on the main network.
Do I just have the wrong idea? If so, what protection does the guest network really offer?

My router is an Asus rt-n66u and is currently set up...

Is my guest network set up wrong? Or do I just misunderstand it?