Not exactly sure, but I know what you are experiencing. It's annoying to have to keep custom JSON and XML parsers available in your favorite REST API calling language (in my case, PowerShell). I, too, would like to see some consistency, but I'm pretty sure what we are seeing is anything XML related was created long ago and never bothered to be updated, while JSON related returns are relatively newer in the product's lifetime.
I also see that with the Body of the API calls too. Some take full example examples while others take JSON in the body of the REST API URI.
More consistency please!
--j