2-3 options. Maybe they got the 230V version delivered by mistake or they figured out just to use 2 phases for "live" and "neutral" + ground???
1300W on 110V household is a pretty heavy load
Or maybe you are not resident in US, and they sent you the right version
i am , in the border with USA. in California.
I think it is 2 phase hot and neutral,
If you look carefully on the 3rd picture, there is AC 1 and AC2. And ground.
For me, its better if its 2 phase, the amp would work better as you say.
I will run it 2 ohm.