Diversity Erasure

 Being the descendant of both notable pirates and Caribbean natives, I often get asked to give an opinion on the Pirates of the Caribbean movies*. Usually, people home in on the sequence with the cannibal tribe, who are possibly supposed to be Kali'na, and ask if I'm offended.
Actually, the movie has a much bigger problem (also, I rather assumed the tribe was more a universally problematic 'Ambiguously Brown Savages' fantasy setpiece rather than anything specific). The pirates are overwhelmingly white, in a time and place where the Afro-Caribbean folks should outnumber those of European descent** 10 to 1, even before you factor in the people of Asian, Native or mixed ancestry. 
This happens a lot in historical fiction, as though people believe non-Europeans were invented in the 20th century. However, modern media can be just as guilty, with improbably homogeneous casts of characters appearing in incredibly multicultural areas. The only route is to do your research (no, watching period dramas doesn't count!) before making any assumptions about the ethnic/religious/racial makeup of a particular setting. 
It's part of human nature to like statements like 'all X are Y', simply because our brains prefer tidy categories to complexity. People also tend to gravitate to those with whom they share some common traits. However, you will have a much more realistic--and much more interesting-- setting if you dig in to demographics (or invent some interesting demographics for your fantasy world) and have a more varied cast. 

*My official, Serious Business opinion is that I have enjoyed them, precisely because they know they're not Serious Business.
**There are Celtic people being factored in to both tallies, so I'm calling it a wash to spare you an explanatory thesis.