It is not uncommon for some Arowana keepers to keep their water level a few inches below the top of the tank. It's to help prevent the fish from escaping in case if they jump.
Some breeders or sellers also do this for young aros. Instead of putting them in a smaller tank to grow out, they put the baby directly in the large tank so they wont have to move it to a larger tank in the future and possibly damage the fish. They will keep the water level 4-6 inches below the top of their tank and gradually increase the water level as the fish grows. If you're wondering why they do this, its so that they can properly feed the baby. Imagine a 1-3 inch baby aro in a 180-300gal tank.
They could easily be raised in tanks without lowering the water level though so basically, its all preference.