I don't think it's normal, but remember the old ex-CAI fares, the difference between DONE3, DONE4, and DONE5 was absolutely negligible. We've also had one poster recently who wanted ex-BRU, in their case the DONE4 and DGLOB4 were identical priced. I know ex-USA fares aren't like that but there's always a chance.

But it wasn't really clear what the OP did to trigger the pricing. Maybe it was actually a DONE4 all along? (Europe to Australia without a transit / stopover in Asia?) The OW tool does weird stuff.