I suggest you have a look at the AP rate table.

Bands are predominantly determined by volume, not location. And zones are, these days, same city, major metro, and rural.

3810 is considered by AP to be outer Melbourne. Reasonable, considering how far it is from the CBD. Narre Warren is closer, thus considered Metro. The line has to be drawn somewhere. And 60km to Melbourne is hardly metro, no matter how much you wish it was.

None of this has anything to do with the bumper not understanding how postage works, except periphally, in that you don't seem to be able to understand it either.