BBUS, page-528

I think because as an ETF Betashares are the market maker here, it is observing the difference between the previous close and open, as such the gap was large so even tho the S&P Futures down on open, ASX based BBUS ETF had the to drop further to correlate, instead you will notice they were only 'a little bit down' rather than 2.3-2.8x down.

Our risk then is when they require to do this and in particular due to the swings they rebalance far more frequently than normal, having to correct pricing which I think might have an eroding effect.