Suppose I'm entering the US after arriving on an international flight. I fill out a US customs declaration form, which includes my signature stating it is complete and truthful. The last part is the sticking point: this means complete and truthful as of when??
The customs declaration form is examined by both a border control officer at passport control, and by the customs officers at customs. Suppose I am carrying with me (either in my carry-on or checked luggage) some item (e.g. fresh fruit) which might be problematic to take through customs. I might decide it's easier to just consume and/or discard it before going through customs but after going through passport control. Say the item in question is food I took on the airplane (hence why it was not discarded before boarding the flight). Given that I don't have the item with me when I go through customs, do I need to include it in my customs declaration? Such a declaration would be accurate as of when I go through customs, but not accurate as of when I go through passport control.
I should note that writing it on the form and then crossing it off once I eat it or discard it is not a perfect solution! It is often the passport control officer who decides whether I must go through extra screening at customs, and having food with me is often the deciding factor for this.