The wire to ground connected to the frame is NOT a return path for current, it is a safety ground designed to blow a fuse/breaker if there is a short. Whenever you see the safety earth symbol it is only intended for use as a path for fault current, or for suppression capacitor ground.
So, the unit is designed for 240VAC power. In your 120VAC country, it is quite common for high loads like this to be run by using 2 phases using a Scott T transformer or so. Each phase will be 120VAC with respect to ground, but they are 180 degrees out of phase between them, so you will measure 240VAC across them.
In a 240VAC country like mine, L1 would be the "live" and L2 would be connected to neutral. This is why you can see L and N used in some of the wiring diagrams - if it was wired to a 240VAC system, this is how it would be wired. It's also why you see the Brown/Blue or Red/Black wiring in places - this is what we use for Live/Neutral in UK & Europe.