It's not just installed, it actually got started based on your process lists. I don't know how bsd and xfce specifically work in this case, but some educated guesses:

- depending on whether you restarted between tests or not, DE shutdown may not clean up all services

- xfce may have a list of supported service alternatives and mate-screensaver was started as the expected-preferred one

- some services may be registered to be stated together with the GUI through the .desktop files, which may explain kalendarc

But as I said - just guessing here.