Why might you lever up the betas of comparable companies differently when valuing a private company?

A private company has no observable beta, so you take public comps' levered betas, unlever each at its own capital structure to get asset betas, then relever at the private company's target (or industry) capital structure and marginal tax rate. Because the private firm's leverage and tax position may differ from any single comp, the relevering D/E and tax rate are specific to it. This produces a beta that reflects the target's own financial risk, not the comps'.