It's almost certainly a sales city thing. Fares for sales city ORD (often the default when using united.com or when logged in to united.com from an account that has most recently searched for travel with a US point of sale) are often different than fares for sales city SYD (often the default when using United Australia or when logged in to united.com from an account that has most recently searched for travel from an Australian point of sale).

I have seen this a lot with a residence in the UK and with valid billing addresses in both the US and the UK.

Note that it's not just fares; fares *and* inventory can vary based on point of sale. For example LHR-EWR may be available in K for a US point of sale but only S for a UK point of sale (no clue why, just how it is; like you will literally see S9 … K0 with a UK POS in the inventory search).

Try changing your 'location' in the top right of united.com (but don't choose Denmark, ha ha) and you'll see how fares vary.