You're right about tyre noise being tested externally — which means the noise (or lack thereof) you hear in the cabin will be affected by tyre pressures, road surface, noise dampening within the car etc. With that said, decibel levels are measured on a logarithmic scale. This means that an increase of just a few decibels can represent a big difference in noise levels. You can see a bit more info on that here: www.goodyear.eu/uk_en/truck/eu-tire-label/noise/