He said there was a "culture and informal pressure of having targets and expectations" in police forces.
Another possibility for the discrepancy was that more low-level crimes were being dealt with informally and outside the formal crime recording system.
Mr Flatley also suggested it was also "possible" that reductions in police budgets and officers meant fewer offences were being recorded.
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Northern and eastern areas will see a touch of frost at first, but most places will see sunshine.