Which is made all the stranger when you consider that Intercity-sector stock was used on commuter runs on several of these lines - and still is, in some cases. At the end of the day it's completely arbitrary.Historical reasons that the industry has never sought to review is the short answer.
Huntingdon marks the NSE/InterCity boundary on the ECML, Manningtree on the GEML, Northampton/Wolverton on the WCML, Didcot on the GWML and Bedwyn on the Berks & Hants.