P & I Clubs set premiums and calls based on risks and claims paid. It is a competitive business, so they are not doing it for fun. So there must be a reason.
I'll check more into it with the brokers in a bit and let you know.
But truth be told, US mariners are a specialty product outside of the US domestic trades and barely register in considerations of most international operators. There are good American fellows out there, make no mistake at all - but we seem to just not get there very often for some reason. American owned and managed foreign flag fleets, or otherwise.
Recruiters, managers, crewing agencies, skill sets, experience base, prejudices, complaint history, injury costs ....... It all plays a part. I wish I had an answer and more American mariners, but it is like the wind is just blowing the other way. No excuses.