Actually, I wasn't entirely truthful. I do think I know part of why it works. It's because we want to believe, that in some universe, love isn't bounded by the taboos of age, and that attraction and devotion can transcend it, which is what I like to think of your Harry and Severus in this one.
The Mary Kay Letourneau brouhaha made me think of this. Bad beginnings, to be sure, and different from the what happens in TDWDYU. But now, with him in his twenties, and they're together, proving (so far) that as wrong as it was it just might turn out to be right. Messes with your mind a bit.
But irregardless of all that, I still love your story.