I just had occasion to call HMRC about some unpaid tax (I changed jobs in April 2012 and got 2 x allowances).Having gone through the voice activated menu and security (NHI No., Full Name, DOB, etc.) I was then asked the same again by the man who answered my call (very quickly I have to say!) and when I asked why this was necessary he said it was because the two systems weren't talking to each other.
Then, when I queried why I had a payment demand 'because they were unable to because...' (I don't have the original demand here) he said it was because they couldn't establish whether my current salary was enough to claw it back! When I pointed out that they now have RTI he GRINNED. I distinctly heard him GRINNING, over the 'phone!