Moz is using its own algorithms/systems for calculating the PA, DA and Spam Score. Their results are based on their own 'numbers' (number of links that they have discovered, the 'authority' that they give to each domain/page, etc...).
To answer your question, according to Moz, your site has probably received couple of backlinks that they consider as spammy. A spam score of 12% is not that bad. Continue with your strategy but from now on try to use more genuine links if possible.