somebol, that is explained in prior posts - expertflyer uses a bank rate (if available) as it seems to be for SGD-USD, but otherwise defaults to the IATA tables (which use a dated rate) for SGD-AUD.

If you ticket fro KRT you will either pay in USD on the oneworld engine (which appears to use a bank rate) or in SGD in which case your credit card company will charge you bank rates (equivalent to xe)