Size of data "for back up" different than "backed up"

On my WD software, it says I have 10,548 files (8.40 GB) for backup.  And this keeps growing everyday.  However, it says for Data backed up I have 10,527 files (8.13 GB).  They were the same on my initial backup the day I installed everything.  Shouldn’t they be the same if it is still backing up everything?

I think Smartware only backups music, movies, pictures and documents and it does not support all the type of files. Maybe is leaving some unsupported files.