oracle datafile size can be greater than maxsize

SQL> create tablespace test001 datafile '/u01/app/oracle/oradata/test001.dbf' size 1M autoextend on maxsize 10M;

Tablespace created.

SQL> alter database datafile '/u01/app/oracle/oradata/test001.dbf' resize 20M;   

Database altered.

SQL>

SQL> select AUTOEXTENSIBLE, MAXBYTES, BYTES, file_id from dba_data_files where file_name like '%test001%';

AUT   MAXBYTES      BYTES    FILE_ID
--- ---------- ---------- ----------
YES   10485760   20971520         13

SQL>
###attention here ,the command runs fine , but the valus of maxsize is not what we expected , it will be the same as 'BYTES'

SQL> ALTER DATABASE DATAFILE  '/u01/app/oracle/oradata/test001.dbf' autoextend on  maxsize 15M; 
Database altered.

SQL> select AUTOEXTENSIBLE, MAXBYTES, BYTES, file_id from dba_data_files where file_name like '%test001%';

AUT   MAXBYTES      BYTES    FILE_ID
--- ---------- ---------- ----------
YES   20971520   20971520         13

SQL>

MAXSIZE is a limit that you can override manually.
the MAXSIZE limits the automatic extension of the datafile but, Oracle let you
the possibility to override this by resizing manually your datafile.