What does fill_container do on a mushroom card?

I’m experimenting with making a dashboard in section view type with mushroom cards. In many sections I have more than one light and so the mushroom light cards tile the section as the default size appears to be 6 units. In some sections I have just one light, which looks odd stuck at the left of the section and only half width. I assumed that clicking ‘Fill Container’ (or setting fill_container: true) would make it stretch to fill the container but … it does not. Messing around turning that option on and off in various places I can’t actually make it do anything.

The documentation says it will “Fill container or not. Useful when card is in a grid, vertical or horizontal layout”, well the layout of a section is ‘grid’ so I expected it to fill it.