Why is it that for cos using PTB valuation, one should buy the co only if the PTB is below ave, (i.e. 1 or 2 SD below ave) but for cos using PE valuation, one could buy at ave PE if co has strong biz model or economic moat? Note that the former type of cos have backing of solid assets like investment properties but latter type of cos is dependent on its biz model to continue generate reliable future earnings.