Server Administration :: Automatic Memory Management

I have read this article:

[URL].........

And i see:

Quote:
If MEMORY_TARGET is set to non zero value:
SGA_TARGET, SGA_MAX_SIZE and PGA_AGGREGATE_TARGET are set to 0, 60% of memory mentioned in MEMORY_TARGET is allocated to SGA and rest 40% is kept for PGA.

So, I have set:
alter system set sga_target=0 scope=spfile;
alter system set pga_aggregate_target=0 scope=spfile;
alter system set sga_max_size=0 scope=spfile;
alter system set memory_max_target=512M scope=spfile;
alter system set memory_target=300M scope=spfile;

, and then bounced the instance.
After startup, I see:

SQL> startup
ORACLE instance started.

Total System Global Area 272027648 bytes
Fixed Size 1384012 bytes
Variable Size 100663732 bytes
Database Buffers 163577856 bytes
Redo Buffers 6402048 bytes
Database mounted.
Database opened.
SQL>

But Total System Global Area should't be in that case 60% of memory mentioned in MEMORY_TARGET? Memory mentioned in MEMORY_TARGET was 300M, and 60% of 300M is 180M, which is not 272027648 bytes.

I just want to use automatic memory management, so I've set the other parameters above to 0. My instance is on my local machine, so just for my own personal use.