When no tablespace clause is indicated, then DB2 choose the tablespace according to the next algorithm:
If this clause is not specified, the database manager chooses a table
space (from the set of existing table spaces in the database) with the
smallest sufficient page size and where the row size is within the row
size limit of the page size on which the authorization ID of the
statement has USE privilege.
If more than one table space qualifies,
choose the table space in the following order of preference, depending
how the authorization ID of the statement was granted USE privilege on
the table space:
1. The authorization ID
2. A role to which the authorization ID is granted
3. A group to which the authorization ID belongs
4. A role to which a group the authorization ID belongs is granted
5. PUBLIC
6. A role to which PUBLIC is granted
If more than one table space still qualifies, the final choice is made
by the database manager.
Table space determination can change if:
Table spaces are dropped or created
USE privileges are granted or revoked
http://pic.dhe.ibm.com/infocenter/db2luw/v10r5/topic/com.ibm.db2.luw.sql.ref.doc/doc/r0000927.html
As you can see, depending on many parameters the tablespace USERSPACE1 could or could not be selected. The best is to indicate the tablespace, or not allow to everyone to create tables in other tablespaces.