But that's what the definition of "selectivity" is for database retrieval. Selectivity == n_rows_selected/n_row_count. The "larger number" was the denominator and the "small number" was the numerator.

Your example SQL is not consistent with your previous sentence:

  SELECT TOP 1000 FROM resumes ORDER BY received_date 

For examples of Harvard and Google, we know the denominators (the total applications and total resumes). Therefore we know the selectivity.

I suspect you're mixing up "mathematical selectivity" from "decision process selectivity" because Google's internal decision tree for hiring might look to outsiders as "black box" or nonsensical.