Absolutely they are. Think of it in this way: assume that there was no deviation in difficulty from Oxbridge all the way down to the bottom of the league tables. If someone that studies (e.g.) law at Oxbridge can struggle to get a 2:1, how on earth would anyone at a bottom-tier university get even close to a 2:1? Considering the fact that Oxbridge A-Level requirements are often A*AA or higher, whereas the bottom tier unis will accept stuff like CCC or worse, it's obvious that it must be harder to get a 2:1 at a higher uni (and therefore easier at lower unis), otherwise there simply would be no 2:1s or 1s at the lower unis at all, because the students there would not be smart enough.