Why would there have been such a gap between enlistment & mobilisation?

Kept back in reserve until required ?

[update]

The Derby Scheme was a voluntary recruitment policy in Britain created in 1915 by Edward Stanley,
17th Earl of Derby. The concept behind The Derby Scheme was that men who voluntarily registered
their name would be called upon for service only when necessary. Married men had an added incentive
in that they were advised they would be called up only once the supply of single men was exhausted.

The scheme was also referred to as the "Group System" as men were classified in groups according
to their year of birth and marital status and were to be called up with their group when it was required.

The scheme proved unsuccessful and was abandoned in December 1915, in spite of the fact that the
execution of Nurse Edith Cavell by the Germans, on 12 October 1915 was used in recruitment rallys
by Lord Derby. It was superseded by the Military Service Act 1916.

215,000 men enlisted while the scheme was operational, and another 2,185,000 attested for later enlistment.
However, 38 per cent of single men and 54 per cent of married men who were not in 'starred' occupations
failed to come forward.

Conscription began during World War I when the British government passed the Military Service Act in 1916.
The act specified that single men 18 to 41 years old were liable to be called up for military service unless they
were widowed with children or ministers of a religion. There was a system of Military Service Tribunals to
adjudicate upon claims for exemption upon the grounds of performing civilian work of national importance,
domestic hardship, health, and conscientious objection. The law went through several changes before the war
ended. Married men were exempt until June 1916, and the age limit was eventually raised to 51 years old.
Recognition of work of national importance also diminished, and in the last year of the war there was some
support for the conscription of clergy.Conscription lasted until mid-1919.

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Ἀριστοτέλης A Gloster Boy in the Forest of Dean ><((((*>

Early in WW1 conscription was resisted by the government, but as the flow of volunteers dried up Prime Minister Herbert Asquith appointed Lord Edward Derby to be Director-General of Recruitment in May 1915. He devised the so called Derby Scheme whereby men registered for service in the Forces but would not be called up until necessary. Married men were assured they would not be called up until the supply of single men had been exhausted. The scheme registered large numbers of men for delayed service but did not supply sufficient immediate manpower, and in January 1916 conscription was brought in. There were men in reserved occupations who were not called up until later in the war when manpower shortages became desperate. In 1917/18 men were conscripted from the Forest of Dean collieries in a process known as "combing", taking so many men of suitable age and fitness from the labour force.
So your grandfather could well have been registered under the Derby Scheme.