I think I see the root of my problem maybe. I'm using Mint 19, which is based on Ubuntu Bionic (18). What you have here is for Mint 18, which is based on Ubuntu Xenial (16).
My PPAs are connecting for bionic updates instead of xenial. So in your above lines of code, my output reads bionic in...
akhavr/dash-electrum and akhavr/dash-electrum (Sources) are both enabled.
They're the ones set up by the installer program.
Linux Mint uses the Ubuntu core.
Not updating from Linux repository
I'm running Mint 19 and for some time now my regular system updates have been checking the DashElectrum repository and installing program (and required file) updates. My last automated update was a couple of days ago, which installed v 3.3.8.3.
Nevertheless...