Spirantized puzzle

This is strange:

Skt. Nsg pitá, Ds pitré (< *pitréi)

Av. Nsg ptá:, Ds fðrái (< *pitréi)

PGerm. Nsg *fáðar, Ds *faðri (ON fæðr)


Why is it that the Avestan Nsg is similar to the Skt. Nsg, but the
Avestan Dsg is similar to the PGerm Dsg, complete with both Grimm and
Verner? Doesn't that call for an explanation?


Torsten