Trouble understanding why Example D gives different results

$0$0 Part 1 of Example D uses EXISTS, while part 2 uses IN. I know that HumanResources.EmployeeDepartmentHistory has duplicate BusinessEntityID's, e.g., the BusinessEntityID of William Vong. I am not sure why part 1 does not show William Vong twice like part 2 does. Is it because the subquery in part 1 is dependent on the outer query, so that each time a row is evaluated in the outer query, it does not matter if more than one result shows up as TRUE? If multiple results match the outer query,