I think there might be a bug on the mirror modifier shortcut. As I use the shortcut (shift W) I get a different result as if I just add the mirror modifier.
Below I shared a link to a video I took for my problem (the empty I was mirroring the object is located at (0,0,0).
Hello. There’s been an upgrade to the Sensei Format “Quick Mirror” feature “Shift-W” in Sensei Format. You can learn more about it in this video: https://youtu.be/VQgouR5dd-U?t=38m15s (starting at around 38 minutes in).
The new feature is also explained in the Sensei Format hotkeys PDF which you can access here on the site, or from within Blender by pressing File > Sensei Format > “Sensei Hotkeys” (on page 3 of the hotkeys I believe). I’ve made a note about this change on the main Crash Course “Mirror Modifier” video page here on the site as well as in the description of the Youtube video (until an updated version of the actual video is released). You can also disable this new functionality in the Builder options menu by unchecking “Quick Mirror Bisect”.