They should line up exactly and if not either the motor drive pulley is misplaced or (more often the case) the fan unit seizes up. Both should be easy to spin when belt is attached!
motor pulley seems too low aprox 2-3mm belt rides on top edge of pulley for short while then comes off, tried to ease plastic pulley up the shaft but could not move it didnt want to force / break pulley, both fan and motors run freely