>>It is not because of autoextend is on,
I hate to disagree but the docs confirm it is...
http://download.oracle.com/docs/html/B12015_01/oracle_database.htm2.25.1 Tablespace Space Used (%)
MaximumSize: maximum size (in MB) of the tablespace. The maximum size is determined by looping through the tablespace's data files, as well as additional free space on the disk that would be available for the tablespace should a data file autoextend.