The manifest was prepared at the port of departure, therefore all passengers booked and traveling were listed. Some passengers did not make the sailing (for one reason or another), therefore you will find their name on the manifest with a line through it. On the page for the Dinnerstein's there are four names "crossed" out and if you scroll down to the bottom of the page you will see the officer indicates there are 26 passengers (of the 30 listed, 4 were crossed out).
On a manifest I researched a few years ago, found the family's name crossed out but then they were listed on another ship just a week or two later. They were detained because of a child's illness.
If the child had boarded the ship with his mother and siblings, his name would have been on the manifest. If he died during the crossing, his name would have been crossed out. At the back of the manifest there would have been an explanation of passengers who became ill or died.
The 1921 manifest lists Michla (age 40), Rochla (age 12) and Leibe (age 9) all going to their father Wolf. Another Dinnerstein arrived in 1921, his first name Morris, age 27. He was the ship's surgeon and not on the same ship as Michla.
The boy may have died prior to Michla leaving Europe for the U.S. and through the years the facts became "mixed up."