Hayo is right, in PM you view the embedded JPEG preview made by the camera, according to your in-camera settings and Canon's colour engine. ACR, on the other hand, not having a way to render the file according to Canon's algorithms, uses its own, Adobe's, engine.
Just do not mind the slight difference. You are after all editing the RAW file, so the output will be different from both starting points. You could of course use Canon's own software, DPP, which would render the file exactly as it was shot (and exact same as the embedded JPEG), but I guess you have a reason to prefer ACR over DPP (which came with your camera after all).
Think of it as different developers, just as with film. Every software looks a bit different.