There’s nothing special about CSS in WordPress. It follows the same rules of specificity that apply everywhere else. This means that the order of rules are can affect how they’re applied.
In WordPress the styles from plugins are typically output after the styles from themes, and styles from Additional CSS are output last. So if you have a CSS selector with the exact same specificity in your theme, a plugin, and the customiser, then the customiser will take precedence over the plugin, which will take precedence over the theme.
So if you need to overwrite some CSS in a plugin from your theme, you need to make the rule more specific.