The roof of most cars tapers towards the rear of the vehicle, it's really common that the rear bar will be slightly shorter than the front bar. If you go for longer 'though' bars then there will be enough adjustability in the bars to accommodate this. If you go for shorter flush ended bars it's really common that the rear bar will be slightly shorter than the front bar to account for the roof being slightly narrower at the rear of the car.
Roof bars