"why would they consider it transatlantic when they don't fly the route over the Atlantic?"

I believe the SQ flight sometimes goes via the North Atlantic over Scandanavia, Russia, then down into SIN.

OTOH, CO 99 always heads north and turns LEFT, so its considered transpacific, even though it rarely goes over the Pacific Ocean. Typically it traverses Hudsons Bay, then the Arctic Ocean, then Siberia, Mongolia, China and into HKG. It just goes more toward the Pacific than the Atlantic.