Because the xx.10 Chester used to call at Milton Keynes whereas the xx.10 Glasgow doesn't? It'll mean that the xx.10 Glasgow will catch up with the xx.07 at Stafford whereas the Milton Keynes call on the Chester service meant that it didn't?
1S58 11:10 Euston-Glasgow today (
https://www.realtimetrains.co.uk/service/gb-nr:C75620/2022-02-01/detailed#allox_id=0) 6 minutes early approaching Stafford (pass 12:27 2 minutes early), 2 minutes early after Stafford, arrived Crewe 2 minutes early at 12:45.
1F15 11:07 Euston-Crewe today (
https://www.realtimetrains.co.uk/service/gb-nr:C75510/2022-02-01/detailed#allox_id=0) on time departure from Stafford 12:24, arrived Crewe 12:41 on time.
[PS The xx:10 Chester used to be a possible service for me to use to Wilmslow, but I didn't like it because it was a Voyager and because it called at Milton Keynes. When the Liverpool services had a Crewe stop added this became preferable to me - non-stop to Stafford plus a Pendolino. The xx:20 via Stockport is slower than the xx:40 to Wilmslow, so in the normal timetable I have possible departures home at xx:00 via Stockport , xx:07 & xx:10 via Crewe and xx:40 direct from Euston.]