the parameter ASM_DISKGROUPS of an ASM instance, specifies, which diskgroups should get mounted, when ASM starts (it does not specify which diskgroups are existing).
However the paradigma, when a diskgroup should get mounted has changed with 11gR2. So instead of mounting each and every diskgroup on ASM start, the diskgroups are mounted, when needed.
So instead of specifying every diskgroup in ASM, the resource, which gets started has a dependency on diskgroups. (You can see this on a database if you do a crsctl stat res -p).
Also the depending ASM diskgroups for a database resource may be modified with srvctl.