OK, I'll use small words.
Everyone who crosses at a port of entry is examined. Maybe they don't catch them all, but they look at them all. They can be counted.
Very few who cross illegally - between ports of entry (across rivers, deserts, over fences) - are examined. The number of people crossing are only estimates - because they are not counted.....
Do you get it now, or do I need to draw it in purple crayon?