I have received a reply from Betfair.

The price shown on Betfair Advantage is the final Back price displayed on the screen at the time that betting on a race is suspended.

I looked at an example where the final trade price on a particular horse was $100 but because everything available at this price was snapped-up at the death, the next available Back price was $85 - it is this price of $85 that is shown on Betfair Advantage. As a matter of interest, the next available Lay price at the time of suspension was $160.

I assume this is the reason why the final trade price can often be higher than that shown on Betfair Advantage.