Usually keeping only five rounds in the Lee-Enfield's magazine instead of a full ten as a matter of official policy was a function of fire discipline, safety, and what was considered proper maintenance of the rifle so as to not cause undue stress on certain components.

The Lee-Enfield, introduced in 1895, was the only rifle in use in the early 20th century by a major power which could hold ten rounds in its magazine, compared to other weapons which usually held only five, six, or eight (a notable exception being the 1889 Swiss Schmidt-Rubin, which could hold twelve). The magazine cutoff, a flat steel plate pivoted by a screw through a slot in the receiver with a small handle on one side for manipulation by the fingers, was originally included in the Lee-Enfield as a means to prevent the rounds in the magazine from rising into the receiver and being pushed into the chamber each time the bolt was cycled, and thus allow the firing of manually-loaded single rounds without taking rounds from the ten-round magazine until ordered to do so. This practice had disappeared from doctrine by the time of the publication of the Musketry Regulations Manual of 1909 (amended 1912), which only advocated for the use of the magazine cutoff to allow soldiers to carry a full ten-round magazine without having a round in the chamber, and as an ad-hoc safety device for soldiers using rifles which did not have them. Paragraphs 264 and 265 on page 103 gave instruction on the use of the magazine cutoff:

264.) Troops armed with rifles fitted with safety catches will invariably set the catch to safety before movement. The use of the cut-off is to be confined in their case to occasions when they are not actually engaged with the enemy, when it may be employed for the purpose of either charging the magazine without inserting a cartridge in the chamber, or to unload the rifle while retaining the cartridges in the magazine. It is never to be used to enable the rifle to be used as a single loader, and is not to supersede the use of the safety catch.

265.) In the case of rifles which have no safety catches, the cut-off will be pressed in and the rifle unloaded on all occasions.

Video on the doctrine of the use of the magazine cutoff post-1909

The 1914 amended version of the Musketry Regulations Manual of 1909 stated on page 93 that;

The magazine will hold two charges of five cartridges each, but should, in ordinary circumstances, be loaded with one only, as the soldier will then retain the power of adding another charge, at any time, should necessity demand. If, when on the line of march, it is desired to charge the magazine without loading the rifle, the top cartridge may be pressed downwards with the thumb and the cut-off closed (see para. 264).

World War I provided significant experience on the use of weapons in adverse front-line conditions. The magazine cutoff of the Lee-Enfield was either not fitted or was deleted entirely with the introduction of an “unslotted” receiver as a wartime expedient, along with other simplifications, with the introduction of the No. 1 Mk. III* model in 1915. It was also noted as early as 1915 that the magazine springs of the Lee-Enfield weakened over time if the magazine was continuously kept loaded with ten rounds instead of say, only five, which could cause malfunctions. The Elementary manual of Musketry Training published in 1915 contained notes of experiences learned at the front in its preface and body. Verbiage was similar to the Musketry Regulations Manual of 1909:

Magazine springs may also become weak, if the magazine is continually kept loaded with ten instead of five rounds. The bolt and magazine must be tested every day to make sure that they are working freely. Many accidents will be avoided if men are never allowed to keep a cartridge in the chamber.

....

(iv) **Loading the Magazine.--**The magazine will hold two charges of five cartridges each, but should in ordinary circumstances be loaded with one only, as the soldier will thus retain the power of adding another charge, at any time, should necessity demand. If, when on the line of march, it is desired to charge the magazine without loading the rifle, the top cartridge may be pressed downwards with the thumb and the cut-off closed. After the rifle is once charged the soldier is responsible that his magazine is refilled at once whenever it is emptied.

The magazine cutoff reappeared on the Lee-Enfield after World War I, either on new-production Mk. IIIs or early Mk. III*s with slotted receivers that didn’t have the cutoff plate fitted during initial production, and were then suitably modified and redesignated as Mk III, or on the No. 1 Mk. V model, but this latter variant never entered wide-scale production. The magazine cutoff went away for good on the No. 4 Mk. I, introduced in 1941.

The practice of only loading five rounds in the magazine of the Lee-Enfield whenever possible was official policy as late as 1937:

Small Arms Training. Volume I. Pamphlet No 3. RIFLE (1937)

Lesson 3. Loading and Unloading.

Para 6: Charging magazine.

i.) Magazine holds two chargers, each of 5 rounds, but should generally only be loaded with one.

The Lee-Enfield remained in limited use with certain Canadian Army units as late as 2018, and a manual printed in 1991 also told soldiers to usually load only five rounds:

NOTES

1.) The magazine holds ten cartridges, however, it will normally only be loaded with five.

1.) Le chargeur contient dix cartouches. Cependant, il ne sera chargé normalement qu’avec cinq cartouches.

Sources:

Musketry Regulations, Part I, 1909 (reprinted with amendments, 1912). London: His Majesty's Stationery Office, 1912.

Excerpts of musketry training manuals.