That will probably get somewhat better once you wear out the original pads and replace them with aftermarket ones. The stock pads are a bit soft. This makes them quiet, harder grabbing (when cool) and they tend to heat up (and fade) sooner than harder ones. In the meantime, just try to remember that the rear braked will normally stop better than you expect.
By all means though, check that the rear pads are clean and dry. A little misplaced chain spray, dust, axle grease escaping past a damaged seal or leaking brake fluid will make a brake "grabby".
