xbindkeys key keeps changing

I'm using xbindkeys (apt list shows: xbindkeys/xenial,now 1.8.6-1 amd64 [installed]). My ~/.xbindkeysrc file contains entries I can use to launch/switch to an app using a shell script named jumpapp (https://github.com/mkropat/jumpapp). Here's part of my ~/.xbindkeysrc:

keystate_numlock = enable
keystate_scrolllock = enable
keystate_capslock = enable

#Raise/Run xterm
"/usr/bin/jumpapp xterm"
    m:0x50 + c:28
    Mod2+Mod4 + t

This uses "<Windows_key>-t" to launch xterm. I got 'Mod2+Mod4 + t' by using the xbindkeys-config 'Get Key' feature.

The problem I'm seeing is that sometimes the "Windows" key is captured as:

Mod2+Mod4

and sometimes it's captured as:

Mod4

and other times it's captured as:

Mod4+Super_L

Most of the time it seems to like Mod2+Mod4. Every once in a while my key combination stops working. I've created 3 .xbindkeysrc files, each with the different "Windows" key combinations. When this problem occurs, I bring up xbindkeys-config, get the "current" Windows key combination, and I copy/paste the right one to ~/.xbindkeysrc and reload xbindkeys.

The only keyboards I use are the built-in one in my laptop, and a Logitech Illuminated Keyboard (USB). I've tried using xbindkeys-config on both keyboards using 'Get Key' and they both give the same results.