Yeah, it's already been answered, it's a mix of pragmatism and likely infection risk or the whole country would be isolating. But it does cause a bit of confusion.
If you had symptoms or tested +tive your dad would also have to isolate, but the contact is one chain link away from him currently.
If the +tive contact is A, and you are B, and your dad is C, and then say D is a contact of C, and > means contact then it works itself down the chain one by one if that makes sense.
A +tive or symptoms > B isolate > C not isolate > D not isolate > etc.
A +tive or symptoms > B +tive or symptoms > C isolate > D not isolate > etc.