That's because fixed tables (X$ tables) on which fixed views are dependant, are populated at different stages of database startup. Sume are populeted in nomount stage, otehrs when you mount the database, and the rest when database is open. But AFAIK no fixed view is dependent on any data dictionary table.
Originally posted by SANJAY_G
But does it apply to all the v$ views? My understanding is some of them are available after startup nomount (from memory), some are available after mount (from memory and controlfile) and some are only available only after database is open (data dictionary)