I think the issue was that the transcript recorded a percentage below 85%, which is not considered first-class by Griffith. I think they'd only award a 7.0 to those with a first-class honours if a) the transcript didn't record the overall percentage or b) the percentage was 85% or above.
I called up Griffith admission earlier this year, and the kind lady informed me that the GPA cut off for 2018 was 6.75. However, if you have a first class Honours, the GPA cut off was 6.65. So I guess a first class Honours doesn’t give you a GPA of 7, but does help lowering the cut off value. Hopefully this information helps