Understanding Society User Support

I am trying to make some comparisons across data for the GHQ-12 (including publications using the previous BHPS) and was confused to find that one report (Davillas A, Benzeval M, Kumari M (2016) Association of Adiposity and Mental Health Functioning across the Lifespan: Findings from Understanding Society (The UK Household Longitudinal Study). PLoS ONE 11(2): e0148561.doi:10.1371/journal.pone.0148561) had mean scores for the GHQ that are significantly higher than in earlier publications about the GHQ in the UK population. In the 2016 paper, scores ranged from 24-26 and from about 10-12 in prior studies. I went into the variables in the US database and it looks like the GHQ-12 answers are scored from 1-4, BUT the GHQ Manual states that one should use the Likert Score from 0-3 where "not at all" is scored 0. I believe that using a score of 1-4 is going to make it very difficult to compare results from the US to other studies unless recoding is done. Can you just clarify why the coding here does not conform to the GHQ User's Guide (Goldman and Williams 1988). Thanks.