spoiler confused about jack's son

So i get that the flash sideways is kinda like purgatory. or kind of like your life flashing before your eyes. That is why everything paralleled the original time line. and they were basically recreating their lives off island. But where does David fit in? If the Flash for Jack was suppose to basically be a reflection on his life why would he have a son? Even Locke says "you don't have a son!"

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