The differences could be due to a few things. Firstly the roof and bonnet may fade quicker than the panels as they are exposed to more direct sunlight. If any of the panels/bumpers are plastic then they will have plasticised paint which may make a difference. You dont say if you have had this car from new, perhaps it has had some paintwork but only on part of the car? Or of course as the previous poster stated it could be faulty paint. T Cut is best for getting the original colour back, but it does not always work.