General Discussion Topics, page-15637

Good question. Perhaps you or someone else can provide the answer.
Firstly the volumes are high and consistent over a period of time.
Secondly, they are always 0.25 or 0.75 of the unit, something that us retail investors have no access to. We can only buy and sell in whole or 0.5 units (above 10c).
The important thing to realise is that we retail investors have little control over the SP.
In the long term it is always controlled by the major institutions and we can only go with the tide, but once you understand that, you can take advantage of it.