Why FDI stock in the same year is different in the same reports?
In note 1 of the World Investment Report 2017 (page 39), it is mentioned that 


  FDI data may differ from one WIR issue to another as data are
  continually revised, updated and corrected by the responsible
  authorities, such as central banks and statistical offices, that
  provide FDI data to UNCTAD.


The last figures are the most accurate ones.