As others have suggested it may have been that the first marriage was annulled, in which case the second marriage is the legitimate marriage and was less than a year before the census.
Or if the first marriage was in a register office and they had second ceremony in church, as Antony says may happen, what if Ralph and Edith regarded the second ceremony as the 'proper' marriage and used that date instead of the first.
Or if the first marriage was in 'secret' without the families knowing maybe they had the second marriage to be legitimate in the eyes of their families. As Ralph and Edith were living with Edith father in 1911, if her father wasn't aware of the first marriage then they couldn't really state they'd been married for more than a year on the census.
All speculation of course. Maybe Antony knows if a marriage is annulled whether there's any record of the annulment, which may help answer the question.
It would be interesting to see that first marriage record, to see if any of the witnesses were family members. Both the Loughboroughs and Granges seem well-to-do so you'd assume there would be family members at the marriage. But if not, it may suggest a 'secret' marriage.
Off topic, Martin, I assume you've already searched newspapers for the Loughboroughs, but if not you're in for treat as there's loads of references; births, marriages, deaths etc. I had a look for the marriages for Ralph and Edith and couldn't find anything but find numerous other family members, including their son.