China and the other vacation worlds only have a custom clear and partly cloudy sky. For China the probability weightings are set to 1 for clear and 0 for partly cloudy. So it is extremely likely that China will use the China clear sky, and unlikely that it will use anything else.

I presume that China will occasionally use the basegame default skies for the settings that it doesn't have overrides for, but I've never played it long enough to prove this.

If you want to use a combination of Bridgeport and China, try using the China files for clear/ partly cloudy, and the Bridgeport files for overcast, custom and Stormy. If you change the probability weights to be equal, then they should all show up at different times randomly.