Avanti hourly service from Wolverhampton timetable change

[Pemberton]

Wolverhampton generally has a weekday hourly Avanti service towards London Euston departing around 45 minutes past the hour. On my journeys from there to Coventry I usually caught the 1845 departure, due Coventry at 1930. In the last week this has changed to an 1850 departure and I've no problem with that 5 minutes however, it now sits at Birmingham New Street for a while en route with a new scheduled arrival at Coventry at 1952, so 22 minutes later than before the timetable change. The other Avanti departures in this direction seem unchaged e.g. 1545, 1645, 1745 and so on. Could anyone with more knowledge than me explain why they would make this change to the previous 1845 train to lengthen it's journey time by what is effect 17 minutes?

 

[Bletchleyite]

My guess would be that it's "holding back" 20 minutes to fall into the path of the following service in the theoretical 3tph pattern from New St, perhaps due to where it calls?

 

[The Planner]

The Glasgow 1M14 runs later as it has a Penrith stop and knocks the Edinburgh train that is the 1850 from Wolves.