Query about RMAN internals

We know snapshot control file is read consistent version of control files. RMAN takes snapshot controlfile before initiating backup. Now yesterday, I stimulated a scenario in my test environment, I initiated level 0 incremental backup in one session, just before completion of backup, I added a new datafile to the tablespace in another session, now I am very surprised to see the newly added datafile in the backup pieces created, please note autobackup of control file was set to off, now my query is how can RMAN become aware of the newly added datafile as it already took read only copy of controlfile? Why i can see the