Seeking v$view information

Hello,

How do the v$views handle sql text?
I have read that sql text selected from v$sql is only sql text still in the buffer cache.
Throughout the day, sql text queried from v$sql can jump from 73 selections to 400 selections (ex).
If the buffer can hold 400 sql text statements, why is it that I sometimes can only see 73 sql text statements?
Here is one of my example queries:

select a.sql text
from v$sql a, v$session b, dba users c
where a.parsing user id c.user id AND c.username b.username AND c.username ! 'SYS'
AND c.username ! 'SYSTEM';

Anybody know where I can learn in-depth stuff about v$views? The Oracle documentation disc does not seem to cut it.

Thank You,

Christopher

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