The AIP is for the sponsorship - confirmation that your husband is qualified to sponsor you. After that, the applicants themselves are examined which triggered the medical request.
Medicals are required for all family members, accompanying or not, as you know. The processing manual gives the following exceptions:
All family members, whether accompanying the principal applicant or not, are required to be
examined unless an officer decides otherwise. Normally, an inadmissible family member, whether
accompanying or not, would render the principal applicant inadmissible. There are, however, two
exceptions to this rule described in R23. The first is the separated spouse of the applicant and the
second is where a child of the applicant who is in the legal custody of someone other than the
applicant or an accompanying family member of the applicant, or where someone other than the
applicant or accompanying family member of the applicant is empowered to act on behalf of that
child by virtue of a court order or written agreement or by operation of law.