premise: Under  A,  for example, the lexicographers provide entries for Adam, Ad feminam, ageism,  Ain't I a Woman  (Sojourner Truth's speech), and amniocentesis .  Z  is limited to three  Zamani Soweto sisters, Zeitgeist (`Spirit of liberty, equality, and sorority'), and Zugassent (a term for male continence as practised by the Oneida utopian community).
hypothesis: The lexicographers offer numerous entries for A, but there are none under Z.