Using PITR Backup and Recovery

["beer@xxxxxxx" <beer@xxxxxxx>]

We recently moved to PITR backup and recovery solution as defined in the documentation.  Our basic setup executes the backup start command, and then takes a snapshot of the filesystem and backups wal files.  However, we have database files ( not wal files ) that change while the system is in backup mode.  This happens during every backup.  Is this normal?  

Any insight appreciated.

-bill