RAF WW1 Colliery Haulier ???

[DebbieDee]

Hi

I have a death cert for my grandmother's twin sister who died age 3 in 1918.  In the Occupation section it says

Daughter of James Assender Royal Air Force (Colliery Haulier)

I am curious to know whether this just means Colliery Haulier was my great grandad's usual occupation before the war  (it was in 1901).  There is no entry for him in the WW1 medals list so I am assuming he was non-combatant but I am wondering if he would have continued with his usual job during the war.  I would would have thought mining would have been a reserved occupation if they had them in WW1? 

It does seem strange though if he was in the RAF and working underground. 

Any thoughts anyone?

Debbie