Why does Siri need to know "which John?" for a reminder or calendar event?
Siri can add attendees to calendar events. When creating an event, saying ‘with John’ will add John as an attendee to the event, shown in the event attendees section in the Calendar app later. If you have multiple Johns, it'll ask which one you mean so as to not get it wrong. There isn't a way to disable this functionality as it's part of Siri's calendar functions.