Apologies for yet another stupid question

My grandfather's number was 26995 and his service record (well, what is left of it) states that he attested on 18 November 1915.
jim1 kindly found the Medal Rolls entry for him, confirming the date and number, but Wilfred was in the 11th Battalion, West Yorkshire Regiment, whereas all other associated numbers relate to the 21st Battalion - Wool Textile Pioneers.
Were these numbers allocated per area of enlistment, rather than date order?
Reason: the only other near number - 26966 - Charles Edward Sheldon - whose records have survived, show that he enlisted 2 March 1916 - some 3 months after my grandfather.
Question: why might Charles have been issued with an earlier number - 26966 in 1916 against Wilfred Appleyard being issued 26995 in 1915?