That's because Spock and Nero did not travel into their own past. They instead travelled into the past of an alternate universe, much like the Defiant did when it slid out from "The Tholian Web" and into "In a Mirror Darkly." That explains all the implicit pre-existing discrepancies between the new continuity and the old--they were never one and the same to begin with.
Oh, it's quite possible that Spock Prime wasn't the Spock we knew, either. He could be from any of a near-infinitude of universes adjacent to the one we we're familiar with. Remmeber that parallel universe Worf was in for a bit where Data had blue eyes? He could very well be from that one. Or the one where Picard actually could make it to Worf's birthday party. Or the one where the Bajorans are antagonistic toward the Federation. Or the one...
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Oh, it's quite possible that Spock Prime wasn't the Spock we knew, either. He could be from any of a near-infinitude of universes adjacent to the one we we're familiar with. Remmeber that parallel universe Worf was in for a bit where Data had blue eyes? He could very well be from that one. Or the one where Picard actually could make it to Worf's birthday party. Or the one where the Bajorans are antagonistic toward the Federation. Or the one...
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