In some countries that use 24 hour time notation it's really uncommon to use that in spoken language[0]. When I say four past four in Dutch, it could be 04:04 or 16:04. So even if you've set the clock to 24 hours, that doesn't mean you would actually say sixteen when referring to 16:xx.

You can set the region in the settings, maybe that influences the way Siri interprets time? (I would be surprised if it does, but you never know)

[0]https://en.wikipedia.org/wiki/Date_and_time_representation_b...