[PyGObject] invisible paths between Gtk.TreeView.get_visible_range() ?

[<c buhtz posteo jp>]

The documentation about Gtk.TreeView.get_visible_range() says:

"Returns the first and last visible path. Note that there may be
invisible paths in between."
<https://lazka.github.io/pgi-docs/#Gtk-3.0/classes/TreeView.html#Gtk.TreeView.get_visible_range>

I have stomachache about the fact that there could be "invisible paths
in between". But the documentation here keeps mystic.
Why is it so? In which cases are there invisible paths?

I need to prevent this situations.