Episode Recap - "The Lighthouse"

Sorry it took so long to post this.. I'm as confused as ever. By now, we know that the Flash Sideways's are filled with subtle differences from the original timeline. The question I have is, Is it because of the nuke? Was it some kind of "buttefly effect" thing that threw off the course of history? or Are they in some kind of parallel universe? Following that logic, was all of the time travel from last season, really "parallel universe travel?

Here's the official recap from ABC:

On the island: Hurley and Jack:

• Jacob appears to Hurley and tells him he has to take Jack and go somewhere. Someone's coming to the island, and they need to help them find it.
• To get him to go, Hurley tells Jack "he has what it takes."
• Hurley and Jack trek through the jungle to an ancient lighthouse.
• Jack discovers his name next to the number 23 on the wheel that turns the lighthouse mirror. When he turns the mirror to that number, an image of his childhood home appears in the mirror.
• Jack becomes angry, smashing the mirror. He wants to know why Jacob was watching him.
• Jacob tells Hurley the people will find the island another way. Jack and Hurley just needed to get as far away from the Temple as possible because someone bad is going there.

On the island: Jin and Claire:

• Claire gets Jin out of the bear trap, takes him to her hovel and cleans out his wound.
• She also brings back Justin, the Other she shot, and demands to know where they are keeping her son or she'll kill him.
• Jin tells Claire that Kate took Aaron and has raised him for the past three years.
• Claire kills Justin anyway -- she slams an ax into his chest.
• Scared, Jin says he was lying. Claire's relieved because if Kate did take Aaron, she'd have to kill her.
• Locke visits Claire, and she tells Jin this is her friend.

Flash sideways:

• Jack has a son named David and is late picking him up from school.
• Oceanic still has not found the coffin. Jack helps his mother find Christian's will.
• David disappears, and Jack tracks him down to a music conservatory audition.
• Jack asks why David didn't tell him about the audition, and David says he didn't want Jack to see him fail.
• Jack confesses that his own father told him he didn't have what it takes, and Jack spent his whole life carrying that around. He doesn't want David to ever feel that way and tells him that loves him and in his eyes, he can never fail.