Close control headspace will improve accuracy. It is especially noticably in front locking actions shooring rimless round. Many reloaders strive to matain only .001 to.002 clearance between the head to shoulder lengths of the chamber and loaded cartridges.
Having proper fit of the bore to the bullet is important also. Military rifles are sometimes designed to use a bullet that is slightly smaller than the nominal groove diameter. There there are rifles know to have an extremely large groove diameter from time to time. To know what conditions you are dealing with you might consider slugging your bore to check the groove diameter.
While saying this can happen with Lee-Enfields seems to offend some it is common knowledge that Germanicon GEW 88s have about 4 different possible bore and groove combinations. So you might want to check your rifle.