This is a typical situation in pricing of airline tickets. The airlines compete on a concrete destination (e.g. London - Sydney or Dubai - Sydney in your case). They charge more for the Dubai - Sydney ticket because they can - i.e. there is not much competition on this particular route and they presumably offer a direct connection. On the other hand there are lots of options if you are flying from London to Sydney (you can basically go via a number of Asian cities like Hong Kong or Singapore). From this perspective it doesn't matter that the longer route includes the shorter one, because you as a passenger can't really take advantage of this fact.

Somewhat related questions:

http://travel.stackexchange.com/questions/4440/do-you-have-to-take-the-second-leg-of-a-domestic-flight

http://travel.stackexchange.com/questions/6004/consequence-of-not-showing-up-for-one-leg-of-journey

http://travel.stackexchange.com/questions/6711/not-flying-the-last-segment-of-the-first-half-of-a-return-flight