unequal route behaviour

[TheFlier]

Why doesn't the same consist behave in the same way from route to route?

example:
class 55 with 13 mk2e TSO carriages
on the newc-york route this consist cruises at 99.6mph using 74% throttle.
on bristol-exeter route I have to cut 3 coaches from the consist (leaving ten) and still have problems keeping 90mph at 90% throttle. I've had to add a class 50 helper engine to use the full 13 carriage consist.

Why is it harder to maintain a speed on the bristol route than it is on the newcastle route? (both are on a flat section of track - no gradients, same season/time of day/weather))