ORA-04031 yet 40% of 'free memory' in V$SGASTAT

Hi,
Here are the results of an attempt to connect to our DB a few minutes ago:
SQL> conn system_at_XXXXXXXXX
Enter password:
ERROR:
ORA-04031: unable to allocate %s bytes of shared memory ("%s","%s","%s","%s")

but AT THE SAME TIME from another session (as SYSTEM) I had gotten earlier I could see this:
select ss.*, round((ss.bytes/&nbytes)*100, 2) prop "FREE ROOM LEFT" from v$sgastat ss
where pool = 'shared pool'
  and name ='free memory'
order by 4;
POOL NAME BYTES FREE ROOM LEFT ----------- ----------- ------------ -------------- shared pool free memory 13929264 42.06

(with &nbytes being the SUM(bytes) FROM V$SGASTAT computed before in NOPRINT mode in the same SQL script) And I finally could connect when the "FREE ROOM LEFT" column above was reading... 17.02%.
What does the value of 'free memory' in this view really show ?

Thanks.

SQL> @version
Oracle9i Enterprise Edition Release 9.2.0.4.0 - 64bit Production PL/SQL Release 9.2.0.4.0 - Production
CORE 9.2.0.3.0 Production
TNS for Solaris: Version 9.2.0.4.0 - Production NLSRTL Version 9.2.0.4.0 - Production
SQL> show sga
Total System Global Area 269486288 bytes

Fixed Size                   731344 bytes
Variable Size             184549376 bytes
Database Buffers           83886080 bytes
Redo Buffers                 319488 bytes