Does anyone have answers to that?

how come? even in a snow filled environment like canada, where roads are sanded/salted?

Audi, and Porsche, are pioneers in developing processes to arrest corrosion. Galvanized sheetmetal is only one part of the process. Seams are sealed and body cavities are flooded with wax to inhibit corrosion. The underbody is coated with to reduce noise and to protect from stone chips. This is part of the reason why an Audi is a premium priced vehicle.

saweet, thanks for the detailed answer!

Over 27 steps in the paint process. Over 30kg of protectants applied. All steel is virgin unrecycled steel to ensure consistent quality.

"VAG uses virgin steel, which minimizes the impurities in the sheet metal. After the zinc coating, they add a layer of phosphate as first layer and then as a basis for the paint. Then the car is immersion primered, using the electrostatic method to make sure all nooks and crannies get covered. Underbody sealer damps sound and protects against chips. Multiple layers of paint are applied over an elastic chip resistant filler. Finally the body cavities are flooded with a solvent free hot wax, then the excess is drained."