I think the idea that ranking correlates perfectly to difficulty is a little bit naive. No doubt institutions do vary, but I very much doubt they base how hard their course is on their rank. Rankings, although OK as a guide, include such nonsense as how environmentally-friendly a university is so they're not great. If you can show me some evidence beyond hearsay/anecdotes it'd make interesting reading.
If I had to guess, difficulty probably has a loose correlation to rank, and if you stuck it on a graph it'd look more like a seismometer readout than a mountain slope. And let's not forget that ranking also equates to higher spend on academic services and probably far better teaching, so you should find it easier to learn more. In that regard, it adds an extra dimension to degrees from different institutions, and makes grades more comparable in relative terms, especially given the way employers select candidates, which is based less on specifically how much you learned at university, or how hard your course was, and more on your ability and potential as an individual.
Think of it like the 'bad school' calculations at Oxbridge, which mitigate lesser results from candidates who went to poor schools but who are, at the end of the day, no less 'able' than someone who went to the best schools. I don't imagine employers actually do this, but that would be my justification for treating two people with different levels of education differently, and not bumping 2:2s from RG universities above 2:1s from polys. There's no guarantee that teaching standards differ any more than course difficulty, but I'd assume teaching standard and course difficulty do correlate, independent of rank.
Employers can't really be expected to visit each university to find out how difficult their course is, and how good the teaching is. So they need an arbitrary cut-off.