Why do I have to uninstall apps twice? On my LG Aristo, I uninstall an app by dragging it to the uninstall interface at the top.

But then when I go on Google Play I find that the app isn't uninstalled, only disabled. What gives?

I'm not talking about system apps at all. I download an app from the play store and uninstall it.
I believe you are running into a situation where the launcher doesn't know what constitutes a system app.  System apps are those that are pre-installed, or bundled with your particular OS.  These are apps like Clock, Calculator, and whatever else OEMs like LG has decided to install on the device.

When you drag an icon to the "uninstall interface" at the top of your screen, this is your device's launcher app providing a convenient way to call the Action Uninstall Package Intent

Since you can't uninstall system apps normally, the launcher instead issues  an intent that essentially "disables" the app - the next best thing the system can do besides actually uninstall.