K3b shows that error only when burning DVD ISO's.
However, if you tick the 'verify' option, it does not deliver that message and finishes normal.
Further more, I have verified that the total bit count is the same for a Brasero burned and K3b burned dvd-iso.
With bit count I mean : open the DVD in filemanager and use ' properties' to look at the volume.
That's how it goes on my hardware.