Is it a big margin? If a difference of 2 digit then I suspect it’s a forex conversion thing.
If you book one way from DPS your ticket will be priced in IDR but a round trip from Australia will be quoted in AUD, but VA’s revenue control probably sets fare in AUD, and so when you compare a round trip to 2 one way, there’s 2 conversion along the way (from the AUD fare in the system to the IDR fare on the website then to your google conversion back to AUD).
At each of those conversion, depending on the rate used and the mark up on those rates you can start accumulating differences. One thing to worth noting is that if you simply use Google to convert that won’t be the actual rate that gets applied by your bank. There’s always a spread in the forex market, and nobody can actually exchange currencies at that exact price quoted by Google or your FX app at that exact moment - you’d always have to pay a little bit more for the transaction.