I noticed that something I had in my Third Homepage Area wasn’t taking up the full width of that section, unlike what was in the Second Homepage Area. Inspecting, I discovered that the Third area is inside the second, and its widget-container width is set to 48%.
I found another thread where you mention that the Third is for 2 widgets across (and the Fourth, I think). So in my case, that explained the issue, and I simply moved my second widget from the Third to the Second.
But I’m wondering how I should have known that detail from the documentation. I can see it illustrated in the Theme layout image, but not explicitly noted. Could that information be added to the Xi documentation page and/or image?