If you buy a train ticket on Sunday in Melbourne, you should mention the Sunday saver. It costs $2.75. However, if you don't mention the Sunday saver, the ticket will cost you $11. The ticket seller will not say anything about the Sunday saver to you. What kind of system is that?
It used to be in the USA that some round trip fares were less than one-way. That doesn't take a genius to figure out.
Also, if you booked for a smaller airport that connected through larger one (e.g., Minneapolis [large] vs. Rochester, MN), it was cheaper to book for the smaller airport. I did that many times and simply got off the plane in Minneapolis.
It all adds spice to life, particularly when the difference in fares may be a whole $10 USD.
A pretty clever one, if you think of it in terms of your local economy. It means that the locals who use the system more frequently end up paying less per trip (assuming they eventually clue in to the deal) than, for instance, tourists. That means that more money is coming into your local economy proportional to the amount of locals using the system (and otherwise required to pay for it).
My advice is to take advantage of it while it lasts, and not make too much of a stink about it, now that you're in the know. Your local taxes probably pay for a significant portion of your transit system - let the non-tax paying tourists pay their fair share!