Doubt in Residuary Guardianship and De facto Custody

Yaar samajh nahi aaya... I'm stuck on this Family Law concept for AILET. Residuary Guardianship and De facto Custody seem like two sides of the same coin, but how do they actually differ? Both deal with the care and control of a minor's property and well-being after the death or incapacitation of the parents, riight? But the problem is, the case laws and judgements are all over the place, especially with regard to Hindu Minority and Guardianship Act, 1956, and Guardians and Wards Act, 1890.

For example, in the case of T.R. Shankaranarayanan vs P. Harnam (1971), the SC held that residuary guardianship is applicable in absence of a valid will or where parents are both dead, leaving no other guardian.