How does Main Toolbar configuration work? (Item visibility)

I have two installations of Idea on two machines (both IntelliJ IDEA 2018.2.4 Ultimate Edition) with seemingly identical Main Toolbar configurations, yet on both these machines the Main Toolbar looks different.
How come so many items aren't visible in one of the installations despite being listend in the Main Toolbar configuration? (See the vastly different toolbars at the top of each image, despite both their configurations being identical.)