In all my international arrivals, I have never seen anyone at customs compare my belongings in my possession to any "list" they have of checked luggage. I hand my blue and white customs card to them, "Welcome back" and I leave. I am sure it is in their system to know, but in reality, I have never, ever seen them check your baggage against any list.
Most people arriving into the US won't see it happen because of how it is done -- even those with many hundreds of arrival are generally unaware of what does occur and with what frequency.
Some people do get flagged for additional customs screening where the customs declaration form is ordinarily turned in -- it's from there that the risk of an issue arising starts to rise substantially -- particulrarly so if the search is for more than a quick "ag" inspection for biological products of the relevant sort.
Also, it's not unheard of for someone waiting at the baggage belt for several minutes to try to leave and then getting themselves flagged by leaving without having claimed any luggage from the baggage belt.