please explain me sga size

This is show sga of a instance.

Question is :
1. I declared shared _pool_ size in init.ora 9MB.

  db_block_buffer =550
  block_size=4096
  log_buffer=32768
all are defined in init.ora file.

Please tell me how come Total system global area is 13028012 bytes.

Where fixed value,database buffers and redo buffers values comes.

Please explain me.

Total system global area     13028012 bytes
Fixed Size                       47788 bytes 
Variable Size                 10653696 bytes
Database Buffers               2252800 bytes
Redo Buffers                     73728 bytes