If you could see the temp. on grid point, it looks like that:

Small islands (like St. Croix) are represended by only one grid point and the temp. at this grid point is "smoothed" with the other nearby points.
Biger islands (like Jamaica) have more reliable data.
Meteorologists try to improve the accuracy by makings models with finest grid but it needs more (much more) computing power
especialy if you want a model with global coverage.