Confusion with capitalisation usage.
On the page:
https://developer.gnome.org/hig/stable/drop-down-lists.html.en
it states to use "sentence capitalization" for both the drop-down label and drop-down items, with a text based example confirming this.
The image in the "Custom values" section immediately below this shows header capitalisation in use for both label and item instead. This may be confusing to some people.