I'm not sure if Marriott is trying to deceive anyone with the way they do this or it's just the system being built one way and no one ever doing any user testing to realize what they're doing.

I say it's probably not the former because often times they'll display a room cost that is super high but it's just for one night of your booking while the other nights are half that price. So you could be booking a 3 night stay and the search page will say "$300/night" and then the total will be $500 since the first night is $300 and the other two are $100.

Like others have mentioned it needs to be an average rate and they should include taxes/fees.