Why show desktop (with alt tab) in kde renders alternatively two different things?

If I use task switcher to show the desktop, I obtain the first time:

The second time:

And so on further. Why?

gsettings set org.gnome.desktop.background show-desktop-icons false

Solved.

Alt+Tab is window selector Ubuntu and Windows but in KDE it shows the desktop. In Ubuntu Ctrl+Super+D will minimize all Windows and reveal the desktop (with icons).

The OP's solution is to turn off desktop icons using:

gsettings set org.gnome.desktop.background show-desktop-icons false

Now there are no icons on the desktop at all, whether you minimize all Windows or not.

Another option is to install KDE add-on; kwin-addons in [K]ubuntu 16.04:

sudo apt-get install kwin-addons

From Arch Linux this user writes:

So in KDE you can change the design of the Alt+Tab application switcher. On my previous installation, I always used one called "Large Icons", which was a Mac OS inspired application switcher that only showed icons. In my installation this switcher is no longer available (Settings -> Window Management -> Task Switcher). I can only select Cover switch, Flip switch, Breeze and Breeze dark. Does anyone know where that nice application switcher went?

The solution (which might be your original problem) could be "Large Icons" as this screen (from the Arch Linux link) shows: