For those that care greenslip prices are set by the MAA
http://www.maa.nsw.gov.au/. A govt body. Basically they say to the insurance companies you can charge between this figure and this figure. Kind of like price is right style.
It is then up to the insurance company how they discount or charge as long as it is in this bracket. Now most insurance companies don't want u25's they obvious charge as much as they can to make you go with another insurer. Some can wear it a bit better so charge slightly less but still near the top of the bracket. Get most of your money.
Some are though now startign to realise this is the age group that will be future customers and it may be time to get them on side to support the ageing population.
You may have also noticed that the number of companies offering CTP these days is heaps less to years ago. They all got out cause it ran at a loss. But with the introduciton a few years back of minimum levels of payment for pain and suffering i.e back pain they have dramtically reduced the number of payments. Hence also why they have come down in proce in recent year. No longer does an accident mean you are set financially for live. You get paid now if you are genuienly hurt.