It's 1.6kW at 115V and up, or 1.3kW between 115V and 100V. The reason they do it is that lower input voltages put more stress on APFC stage which is already massive and it's not economically viable to beefen it up even more for the handful of countries using input voltage below 115V (Japan and some countries in Central and South America). For example, basically the only direct competitor to AX1600i - Super Flower Leadex Gold/Platinum/Titanium 1600W and it's rebrand - EVGA G2/P2/T2 1600W are both rated for 115-240V, i.e they don't claim working capacity under 115V at all. Even if technically, just as AX1600i they would work without a hitch if the ambient temperature is low enough, they just can't be sure that it's safe as they wasn't designed to be operated as such.
As of OP's question, it's obviously not 230V only unit, 120V is completely safe, at full capacity.
PS: Also, if it's CP-9020183-CN, - that's a unit for Chinese market (even if it came with non-Chinese plug as these can be just dropped in by distributor), they use 220V only, so they probably didn't even bother putting derated wattage on the label, although internally it should be identical to worldwide version, at least i fully expect it to be.