"Interference", "Jamming", droid targetting, etc.

[Balrog]

Yeah, it is the distance/time the missiles have to make the turn that has always struck me as why it was such a hard shot.

When I watched the movie, I always read it as the torpedo being "sucked in" to the port, because that's what it looks like. Doesn't the failed torpedo hit beside the port or something?

Considering it's an exhaust port, I'm failing to see how or what could "suck" it in, but yes the failed shot did hit beside the port. IIRC, the Proton torpedoes they use in the attack are mentioned in one of the tech guides to be accurate to within three meters, which probably contributed to why it was considered a difficult shot.