Board Thread:The Reference Desk/@comment-82.20.113.152-20130213135009/@comment-6433721-20130406054010

The difference is actually pretty simple.

The Doctor's Universe, from year 0 to Utopia, was completely replaced by what the restoration field extrapolated modified by what Amy remembered as the Doctor's Universe when the Big Bang Two happened. There was only one set of space-time coordinates to be defined as the Doctor's Universe before the event, and a slightly different set of equivalent points afterward.

Pete's World is a full parallel. If you add the coordinate set of the universe of Pete's World to the same of the Universe of the Doctor, you have twice as many points as either. Every point in space-time (probably up to 11 dimensions) has either an equivalent or a replacement value. It's two different universes, not one.

Now, I think you're confused because the different things that could have been changed in the Big Bang Two each were decision points that can create a parallel. Thing is, that's the same of any choice, regardless of how minor. The fact that a different choice creates a parallel does not mean that the equivalent event no longer exists in the home reality.

Ok, I guess it got complicated there, too. ;)