Actually, the difference between XMP-RPC and REST is not just in the format of the response, the difference is the whole approach. The way you have described (didn't go into the docs) you don't have REST at all, just XML-RPC that returns JSON. Sorry if I misunderstood.
There are two different interfaces for the same implementation. Let me stress the different part. The RESTful API uses JSON to encode the responses, but that's all.
Sorry if I didn't explain it very well, the docs are clearer though.
The point is that your RESTful API is not RESTful at all. It's just RPC using JSON. REST is an architectural style that models the interaction around data and hypertext instead of actions.
A good rule of thumb is: if the documentation mentions "methods" as anything other than the uniform ones (if you're using HTTP, that would be GET, POST, etc), then it's RPC, not REST.
Note: I'm not saying your API is bad; RPC is a completely valid model. I'm just saying it's not RESTful.
