Doug.
It's got to do with the way the weight is distributed. The A380 has 22 Wheels to distribute that weight over the pavement. The 789 only has 10. Tyre Pressure also comes into the equation.
I agree that this will not be an issue. Currently in Broome the pavement Limitation can be exceeded by 4 tonnes anytime, all that is required is a phone call.
There are some taxiway and apron works going on over on the T3 and T4 Side at the moment. Maybe they are going to strengthen the pavement over there to elevate the issue.