Do you basically have two plugs coming to your AC power panel? One that can be plugged into the inverter, the other into shore power? Sounds like you are running power into an outlet. That doesn't mean plugs (male contacts) hanging out with power on them does it?

I'm betting that code calls for a lockout relay so that both can't be supplying power at once no matter how you mess up. Our boat is set up that way. The relay hums annoyingly, but we don't have to rely on connecting things correctly to keep things safe. Yeah, more money is involved, but your insurance company might like it better.

I realize that doesn't answer your question. There could be a difference in trigger sensitivity as Al says. I have heard that GFCI outlets sometimes need to get replaced on boats. I think the salt environment has something to do with it.