Some PhD advisors often simply turn down being a co-author, even if they contributed some crucial help or idea. Sometimes it is modesty or generosity, sometimes

Some PhD advisors often simply turn down being a co-author, even if they contributed some crucial help or idea. Sometimes it is modesty or generosity, sometimes a gentle way to say the work is not very good :) More seriously, the solution is not to use content-oblivious measures (e.g. number of papers/citations) when evaluating researchers.