Mendeley citation standardization
Mendeley attempts to extract metadata (title, authors etc) from article PDFs by searching through the PDF content. Since different publishers format their PDFs in different ways, sometimes Mendeley gets more information (e.g. a DOI) and sometimes it gets less. This is not a problem unique to Mendeley – Zotero has struggled with it, too.

There is an article on the Mendeley Support website called 'How does the automatic document details extraction work?' that explains the process in slightly more detail. 

Automated metadata extraction is likely to remain imperfect until publishers consistently produce PDFs with standardised data. Several organisations have made progress on this, but it is not yet a solved problem. Until then, if you want good-quality references then it may be best either to generate them manually or to edit Mendeley's automated references by adding missing information.