It is random application and interpretation, I believe. As linked above, the people being charged $1100 were evidently the owners of the boat (same names as on the ownership papers), yet the BVI still classified this as a "charter" type entry because there were others aboard who had different names (friends of the owners) and charged accordingly. I think that had only the owners been aboard in that case it would have gone through as a private trip. I think that, in Spanishtown at that time, they were looking for any reason at all to charge fees. The French owners were not aggressive or impolite, at least until they'd been presented the bill!