As others mentioned, the road wear scales with the per-axle weigth. With that in mind I checked out the website and I'm puzzled.

Up to 3 axles, indeed the tax is LOWER if the same weight is over 3 axles rather than 2. Well done, makes sense.

But then, at the 4 axles table, suddenly the prices for weights between 12000 and 20999 kg are all higher than their correspondents in the 2- and 3- axle tables.

What are they try to accomplish here? "Don't put a 4+ axle vehicle on the road unless you will carry at least 21 tons with it?"