Why might official police statistics on crime not be completely reliable?

The fact that official police reported crime statistics may not show the full picture could be down to the  'dark figure of crime'. This is crime that hasn't been reported or found out. This hidden aspect of crime is unkown by the police and therefore is not recorded in statistics. 

However, the police as an institution has been influential in society for many years and therefore will provide a huge amount of data. This can allow researchers to be able to conduct comparative and longitudinal studies.