Understanding Email Validation Discrepancies with ZeroBounce and Clay

Can you help me understand how an email validateor like ZeroBounce can return a higher % of "valid email" for contacts who are no longer at that company? I made a list for a Sales rep with 138 contacts with a validated email through Clay and ZeroBounce, but only ~50% of those contacts still work at the company in the "valid email" address.