log buffer

Hi,
On 11.2.0.4 on Aix

We have:

SQL> show parameter log_buffer

NAME                                 TYPE        VALUE

------------------------------------ ----------- --------------------------

log_buffer                           integer     10485760

SQL> show sga

Total System Global Area 2137886720 bytes

Fixed Size                              2248080 bytes

Variable Size                    1157628528 bytes

Database Buffers                956301312 bytes

Redo Buffers                         21708800 bytes

My question is: why size of Redo Buffers and log_buffer are different? Aren't they the same?
Unless mistaken:The initialization parameter log_buffer determines the size of the redo log buffer.

Thanks.