There could many, many alternate timelines and universes as was seen in TNG's "Parallels". We can say that all of those endless variant possible histories are equally valid and "real". We can further speculate that the "Prime" Spock seen in ST '09 may not actually be from the timeline of TOS through VOY that we are used to, but rather one of many possible versions of the 24th Century. In this case, one where Romulus is destroyed and he travels back to the 23rd Century. Not being a knowledgeable scholar of temporal physics, JJ Abrams would have no way of knowing which "future" Spock it was that arrives back in the 2200's.