Hello Tom Rosenthal,
Welcome to Microsoft Q&A forum.
I understand your confusion when the results of the questions you search on Bing might not match what you see on others search engine. If possible, can you please send feedback about the results to Microsoft so they can help improve your search experience?
Regarding your questions, I have checked the link of the questions you asked, as I see, the answer cite the Wikipedia page, where it states Home Alone is the highest grossing Christmas movie franchise of all time, which I believe that cause this confusion since the question is not clearly mentioned it. I have tested a search for "Which standalone movie holds the title of the highest-grossing Christmas film of all time?" and the results is correctly the Grinch. So, this is just wording difference between search engines that cause this confusion. I hope this clarifies.
Best regards.