Like other rifles, Lee-Enfields can perform well with good boattail bullets dimensioned to fit the throat and barrel. If undersize for either or both, they have a greater propensity to yaw and engrave off-axis, because they have relatively less bearing surface than flat-base bullets of the same weight and a portion of their mass lies aft of bearing. A bullet launched off-axis is unstable and detrimental to accuracy.
The basic idea.
Since some L-E barrels and throats are significantly oversize for commonly-available commercial bullets, yaw is a problem when such projectiles (especially boattails) are used.