Asymmetric CLC timetable question

[MancMetro]

The EMT service from Manchester Oxford Road to Warrington Central (nonstop) is timetabled to take 16 minutes. However, the return service takes 20 minutes.

Does somebody know of a reason for this? Perhaps there's a downwards gradient towards Warrington, or the speed limit is higher on the westbound part of the line? Or maybe there's an operational reason?

It's similar with the TPE service which takes 18 minutes westbound, and 20 minutes eastbound.

 

[The Planner]

Those are the public timetable times, the Manchester bound trains have 2 minutes engineering allowance in them and occasionally a bit of pathing time too so that is where the difference comes from.

 

[TheGrew]

I would suspect it is because there is often a queue on approach to Oxford Road. My wife who commutes from Newton-le-Willows often says they should add a stop at Deansgate on her morning train considering the amount of time it sits on the platform waiting for a green signal.