I think you give Siri too much of the benefit of the doubt when you say that Siri understands your request. You may be correct, but maybe Siri just looks for a location like "McDonalds" and most of the time just shows you a list? It's easier when you're ignoring the rest of the sentence as if it has no relevance to improving your response.
It's very hard for us to discuss Siri's internal state; by contrast it's easier to discuss Google's observable performance, which is to semantically differentiate between these two requests.
You can just ask Google, "Nearby McDonalds" and you'll get a list. "Go to the nearest McDonalds" and you get navigation.
