An inner join between two tables does a complete join, it checks for matches and returns rows. This means, if there are multiple matching rows in the second table, multiple rows will be returned. Also, when two tables are joined, columns can be returned from either.
With an IN, what is done is a semi-join, a join that checks for matches but does not return rows. This means if there are multiple matching tables in the resultset used for the IN, it doesn’t matter. Only one row from the first table will be returned. Also, because the rows are not returned, columns from the table referenced in the IN cannot be returned.