This is concerning PubMed Export plugin. OJS 3.1.2.
Normally, the same number is assigned to the authors’ names if they come from the same affiliation. However, in PubMed export plugin, an email address for all the authors is added to the Affiliation field in an exported XML file. This makes all the affiliations look different for PubMed, and they assign them different numbers.
I suggest an email address be included in Affiliation tags, in exported XML, only for corresponding(primary contact) author.