One time when I was specifically flagged by an immigration agent at LAX for a little extra attention from Customs it was by circling the little round DHS logo logo in the upper left hand corner of the 6059B form. I presume that is an informal little signal in that specific local. The customs guy took one look at it and sent me strait to secondary where they did a somewhat cursory bag search.

I agree the numbers are just how many people in the family.