I'd guess that since the radar beam used to image the asteroid was not configured when it was sent nor were the returns read in a manner that was expecting the moonlet, it just got a bright return indicating it's presence, but didn't get a proper "picture".
Also, the moonlet is probably less spherical than the primary, so it's facing side is "pointy" in relation to the radar beam, giving a single bright return. It looks like you can see some sort of shape or faint edges to it, which might be some very weak sub-optimal returns.
The size/mass of the moonlet was probably calculated by measuring it's motion, and that of the primary.
