A default tablespace is just that, a default. If you specify a different table space when you create a table, it will be created in the table space that you specified. Although this usually does not make sense, you can place each table in a different table space so that the user has 20 tables in 20 different table spaces, none of which is the default tablespace .
You can also change the default tablespace of a user. It is therefore possible that the 20 tables were created without a specified tablespace, but when the user's default tablespace was different. Then, the default table space of the user was changed after the creation of all the tables.