The NEXUS machine prints out a ticket but the agent isn't necessarily aware that you're the person it was issued to. The ticket can then be matched to your passport and card you filled out to confirm the identity. It's a matter of judgement as alway.
That might be true with places like YVR or YYC where they can't see you do the kiosk thing, but there are places like YOW (where the kiosk is about 10 feet away and rarely more than one person lined up) where it's ridiculous (and, unquestionably, been carded the most).
Given the YOW thing, I think asking for the card is more about trying to bust someone and revoke privileges.