Just to add further comment to Rob’s explanation:
If for some reason the rates for the year are levied late eg 3rd October (Supp or Split or whatever) & the person becomes eligible for the PCC on 2nd October, then they are entitled to a FULL rebate for the year.
The proportional year rebate only applies if the rate notice is issued prior to the person becoming eligible for a PCC.
Hope that makes sense.