Please correct me if I 'm wrong but I believe UA likes to price all partner (but not JV) tickets based 100% mileage accrual, so these tend to be higher fare classes than if you were buying straight from said partner.
I see that JV partners like ANA actually pricing is very similar if not identical to UA, but other carriers like OZ will be a lot more expensive on UA website. I'm guessing SQ doesn't have JV with UA?