There is probably no mistake.
From 01/10/1795 on France did incorporate what's now Belgium until 1814.
Around 1798 it was forbidden to baptise children. So people did _sometimes_ register their children when they where born and asked a undercover priest to baptise their child when it was possible.
This period is also famous for the armed resistance against the occupant in these regions (the 'Boerenkrijg' = Peasants war).
It was also usual to try to forget to resgiter sons, so that they couldn't be drafted in the army when they were 19 years old.