8.5gb discs only 7.9gb?

I wanted to burn a few 8.5gb iso games for my jailbroken Xbox360, and bought an extra 8.5x DVD + R blanks. I could not burn it with CdBurnerXP, however, since there were only 7.9 on every single disc. Lying on my burner?

This is normal and is not due to your burner. Again, these are the typical differences between the GiB and GB standard systems.

See:

https://de.wikipedia.org/...ffstechnik

You have the typical dual layer blanks of type DVD-9 (writable on one side).