I don't know this for a fact with that movie, but a recent trend is for a studio to submit a film for a cut 15 at the cinema but reinstate the cut material for an 18 DVD. They also do it with cinema 12s and DVD 15s.
I recall reading somewhere that the bulk of cinemagoers are 13-18, so commercially it makes sense to get a lower 15 certificate for the cinema where it's harder for people under 18 to get in and see it but have an 18 on DVD where it's easy for under 18s to see it.