According to a former UIDAI official, the gap is mainly due to Unregistered children and unupdated deaths.

Source: https://www.mutiny.in/india-is-on-track-to-become-the-worlds-most-populous-country-but-how-many-people-it-has-is-unknown/

In case even Proof of Relationship document is not available, UIDAI provides the resident to submit a self-declaration by the HOF in the UIDAI prescribed format.

Source: https://www.siasat.com/uidai-enables-head-of-family-based-online-address-update-in-aadhaar-2494347/

In case even Proof of Relationship document is not available, UIDAI provides the resident to submit a self-declaration by the HOF in the UIDAI prescribed format.

Source: https://www.siasat.com/uidai-enables-head-of-family-based-online-address-update-in-aadhaar-2494347/

The Aadhaar facility for such individuals will be available at selected centres, including the main post office of each district and other Aadhaar centres identified by the UIDAI.

Source: https://www.freepressjournal.in/india/aadhaar-update-passport-like-physical-verification-now-compulsory-for-those-above-18-years-says-uidai

While Aadhaar is issued by the Unique Identification Authority of India (UIDAI) to a resident of India, PAN is a 10-digit alphanumeric number allotted by the IT Department to a person, firm or entity.

Source: https://www.dailyexcelsior.com/linking-pan-with-aadhaar-cbdt-chairperson-says-48-crore-linked-so-far/