Why the huge shmmax default setting?

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Fresh load of Centos6/64 from new ISO (downloaded 2 weeks ago?) and 
getting set up with PostgreSQL, one of the typical steps is to increase 
shmmax from its normal, conservative value (eg: 32 MB or something) to 
something far more aggressive.

But in recent installs of CentOS 6, this value is generally huge, 
typically larger than the RAM installed on the machine! For example, 
fresh installs on systems with 32 GB of RAM, this is already set to 
68719476736 (64 GB)

This large value doesn't really make sense to me - can somebody explain 
why the change to such a large value?

-Ben
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