Backing up from 0 needed to 30 of 60 GB within moments

A few moments ago my PC was fully backed my mbld.  Now it says 30 of 59.13 backed up.   I did not add anything to my PC hard drive in the interim.  How can this possibly happen? The same thing with about the same numbers seems to ahve happened before.  Does anyone have any idea what could be going on?  

I truly find my MLBD to be a myserious black box.

What software are you using?

Thanks for asking!

I am using WD Smartware.  Is there something better than this that I could use?