Prompted by a comment from Socks, I have looked at some of the GB regions where Ordnance Survey road names don't match the names in OSM. The OS Opendata names on roads are provided by OS StreetView and a textual version is provided by OS Locator. ITO World and Musical Chairs both use OS Locator to create a graphical tool to help people identify where the OS & OSM names for roads differ.
Sometimes the name OS has for a road does not match the name displayed on a name board for the road. OSM uses the name from the name board. To help with the processing of anomalies the wrong, OS name can be added as a not:name tag. The occurrence of the not:name tag is a useful indicator to the origin of the names in OSM. If the names closely match the OS list then the OS names may have been used and the actual names may not have been surveyed. In this way the errors in the OS datasets find their way in to OSM.
One possibility is that mappers in an area may not use the not:name tag. They may not like it or maybe they don't know about it. If they survey their road names and don't use the erroneous OS names then these anomalies should show up in any comparison.
I looked at some of the data about OS Locator and threw some data into a list for the most complete 250 authorities. ( I lost the will to live after 250.) You can see the list at osloclist.raggedred.net. If you click the headings it will sort that column, allowing you to see where how each authority compare.