There's a centering hub sticking out from the middle of the rotor. If the center hole in the rim isn't bored to the proper size, the rim hangs up there, instead of sitting properly against the flat mounting surface of the rotor. Get it?

If the wheel's center bore isn't the right size, then THAT is what you need to deal with. New rims have to match the bolt-circle AND the center bore diameter. Hubcentric rings are used when the center bore of the wheel is too big for the hub. If the center bore is too small, you can *MAYBE* get a machine shop to enlarge the bore, but that's not good unless you can find out for sure that it won't damage the strength of the wheel.