Does tables under schema uses multiple tablespaces other than default tablespace
A default tablespace is just that, a default.  If you specify a different tablespace when you create a table, the table will be created in the tablespace you specified.  Though it would generally not make sense to do so, you could put each table in a different tablespace so that the user owned 20 tables in 20 different tablespaces none of which were the default tablespace.

Alternately, a user's default tablespace can be changed so it is possible that the 20 tables were created without a specified tablespace but when the user's default tablespace was different.  Then the user's default tablespace was changed after all the tables were created.