There have been documented cases of NYPD not wanting to take reports. Without crime being reported, they can manipulate the figures to make themselves and the city look better. [typo edits]

Edited to add:

Start here: http://www.villagevoice.com/2010-05-04/news/the-nypd-tapes-i...

Or take your pick: http://www.villagevoice.com/related/to/New+York+Crime+Statis...