Oracle Database Discussions

I am running a test db oracle 11gR2 on windows server 2008.

I am running oracle suggested backup strategy for my backup operation.

So every night i run incremental backup and apply the previous incremental to image copies.

I am confused on why when i run a query like

select file#,name,unrecoverable_change#,unrecoverable_time from v$datafile

i have atleast one or two datafiles whose unrecoverable_change# and unrecoverable_time value is not 0.

Whether the operation was in nologging or not, since i have a incremental backup, why is the above value not 0??

However when i run in rman,
report unrecoverable;

rman does not report anything. So whats the difference b/w rman unrecoverable and the one from v$datafile??

can someone help me understand the correct concept of unrecoverable_change#??

Thanks.