The Transport Minister Lui Tuck Yew has explained how the system work. See his note on his facebook page. On paper, it is a very good system. In real life, it is getting worse. It is inevitable with policies that they decay because the environment which they were first crafted change. Also all policies have undesirable and unwanted side effects, which often accumulate over time. Worse, various policies interact with each other making problems very complex to tackle. Hopefully not, but sometimes you end up staring down at a Gordion Knot and that only way to resolve it is to bring Alexander's sword and brutally cut it.
This is a reasonable formula (from the minister's facebook note) to guide transport fare change.
(0.5 CPI + 0.5 WI -1.5%)
In theory, if CPI or WPI is negative, transport fares could even go down when there is deflation and pay cuts. In real life, I don't remember it had ever happened. It is always up. Lucky if there was no hikes for a long time.
In real life, with widening a income gap, for a growing number of people the CPI and WPI make less and less sense. So the formula need to be changed. E.g., the proportion of a person's income that goes into transportation is more meaningful.
There is no point upgrading the transport system to make it look good and comfortable if people's pockets are hurting very badly. Similarly I noticed many upgraded HDB precincts are more beautiful that the inside of many homes. What for? These folks don't live or sleep outside their apartments!
Politicians and policy makers no matter how smart they are, if they are disconnected from the ground cannot serve the people successfully. They use their academic and model-based thinking to substitute for common sense. Then they issue and rate their own report cards yearly. We will give them a more honest and fairer score at the next elections.