Headcodes starting with "9" also apply to TPE Liverpool-York-Newcastle-Edinburgh trains.
Probably to distinguish TPE trains heading west to Liverpool rather than south to Manchester Airport, at the Ordsall Chord junction.
(Actually it can't be that because there are some "9" trains to/from the Airport too...).
There's no reason to retain the inter-regional codes now the regions don't exist, but they still seem to be used like that in places.
There are some odd ones like on the Marches line, it's V southbound but W northbound (not M).
On the other axis through Shrewsbury it's I towards Birmingham, and J towards Cambrian/Chester (you'd think they would avoid using I and O).
NR route/region boundaries keep changing too, as signalling areas expand.
The Midland main line south of Chesterfield is now in LNE territory, for instance (where trains to Corby use code M!).
With Eurostar there was some complicated reason why train numbers starting "9" also worked for Eurotunnel and SNCF/SNCB control systems.
Presumably it also works in the Netherlands too.