Two Plot Problems

[dumbledore11214]

No: HPFGUIDX 187643

> Carol earlier:
> > >  As for all the problems involved in Lupin's saying that Sectumsempra was Snape's signature spell when he didn't even know that Snape was a DE,
> > 
> > Mike:
> > Please refresh my memory, where did we find out that Lupin didn't know Snape was a DE? Obviously it has to be before GoF, because we all knew by the end of GoF that Snape was a former DE with the dark mark on his forearm. 
> 
> Carol responds:
> In GoF, Sirius Black makes it clear that he doesn't know Snape was ever a DE, and if he didn't know, Lupin wouldn't have known, either. <SNIP of almost whole post>

Alla:

I do not understand it. Are you saying that Lupin and Sirius must know the same things and not know the same things?

Putting aside the fact that Sirius spent twelve years in hellish prison and Lupin spent twelve years we do not know where, but definitely not in prison, which to me already implies that Sirius would not have known a lot of things in the outside world and Remus would have known them, how does that work out exactly even if Sirius was not in prison?