I had a look at the 1911 census image on Findmypast which has:
Name, Relation, Condition/Yrs married, Age, Occupation, Where Born
COCKRAN, Alice Wife Married 5 years [Children: None] 38 Isle of Man Ramsey
[Also, crossed out: COCKRAN, Samuel John Head Married 5 years 52 Ship Steward Worker Lancashire L'pool]
RG number:RG14 Piece:22658 Reference:RG14PN22658
Registration District:West Derby Sub District:Wavertree Enumeration District:5 Parish:Wavertree
Address:
63 Methuen St Wavertree Liverpool County:Lancashire

It was written and signed by "Alice Cockran" and the 5 years married was clearly written. As he was a ship steward they could have married anywhere! Even on board ship.

It looks as though Alice filled in the form putting her husband's name first, and because she signed, the enumerator crossed out her husband's name, or the person collecting the forms told her to do so because he wasn't at home that night.
This puts a totally different slant on the entry than where you wrote "In the 1911 census they refer to themselves as married five years".
There was no "they", and what Alice wrote was absolutely clear, so no indexing or enumerator's error.

Have you followed his career to find out which ships he was employed on and where they went?
Also, where did the second name "Mary" come from? Was this on her death cert in 1912, or does your Subject heading refer to something other than her death? If her death, was her maiden name given there?

Sue