Use a backpack or travelpak that straps onto your back instead of a rollaboard.
Yes it is possible to get 7kg in a travelpak and not go over the weight limit.
Wear your heaviest clothes and shoes on the plane.

Rollaboards are heavy empty and are not flexible when putting in overhead bins.
I got my travel pak at REI and is very lightweight empty and are the same measurements as a rollaboard and gate agents have never asked to weigh my bag but they always look twice at rollaboards because they know those are heavy.
So like others said it is how you pack and what kind of carryon you bring you that makes the difference. I often see a scale at the qantas gates at LAX terminal 4. Just weigh your bag at home on your scale to make sure you dont go over.