i read an article last week that said originally the makers of az recommended 12 weeks. it also said that this is actively under review and that some countries have varied it from 6 weeks to other weeks upto 12.
it also said there is uncertainty over the optimal wait time for the 2nd dose and more research is needed
the bottom line is there is uncertainty, lack of good data and well basically , much like a lot of things with coronavirus, nobody really knows. a lot is really just best guess (albeit by medical experts)