It’s a feature of Kagi. Putting the question mark does invoke AI summaries.

https://help.kagi.com/kagi/ai/quick-answer.html

I know for the summary.

What I say is that the search results part of the page, with or without the summary should be the same in theory.

So if the other person saw a difference in the result returned it might be only because of the impact of the question mark character itself on the search index