The premise is the Maker (Ultimate Reed Richards) has manipulated events in some other universe so that a lot of heroes' origins haven't come to pass along with dividing the world amongst IIIRC a dozen or so people aware of his existence.
In Spider-Man's case it means he's a civilian in his 30s, married with MJ and 2 kids. Uncle Ben is alive and best buds with JJJ.
And it's just been a great comic overall. Best out of the 3 Ultimate books we've had so far