Wild Skies picture of a Blanch is an example of a normally executed 'swamped rib'. The 'shield' (the often scalloped patch that aligns with the top rib - in this example, a rectangular raised block) forms the rear site and when the gun is mounted one only sees a small part of the rib nearest the muzzle.
It is not done 'wrong', it is just a different, and fairly common, design feature.
The Purdey that skeettx posted is another example of the same although I would hazard a guess that it has been re-barrelled or re-ribbed with a swamped rib rather than originally manufactured as such.
Don't forget that changing the rib to suit a new fashion (possibly as far back as the 1890's) may have made an ordinary gun more saleable.
Whether the barrels are original Purdey and fitted by them or not is a whole other bag of worms and probably only Purdey are in a position to comment. I think it unlikely that Purdey would have fitted any barrels that so patently don't fit but money talks and if an important client told them that that was what he wanted done, I have no doubt they would have done it, probably with much forelock tugging!
However, I think it more likely that IF they are actually another set of Purdey barrels, they were fitted on by someone else within the trade less punctilious that our friends at Purdey!