the amounts they put on hold on our card were uneven too
That is indeed very strange. I check my credit cards frequently on-line, especially after a cruise or other travel, and I've never seen uneven holds like those the OP and you have encountered. But I haven't cruised with Princess for a while, with all of our cruises over the past 5 years being Carnival and RCI. They don't do it, at least not that I can tell.