A reply from the UN research office:
Dear Mr. Thirdpower,
The UN and Interpol collect data based on different defitions, which are both correct but do not match, thus the two series are not comparable.
Furthermore, since every country collects crime statistics based on
domestic legislation, some countries find it easier to adjust their figures to the UN definition and some to the Interpol. In some cases, countries do not adjust figures and may provide the same statistics to both data collection series.
In order to improve comparability, the Tenth UN Survey of Crime Trends and the Operations of Criminal Justice Systems
(http://www.unodc.org/unodc/crime_survey_tenth.html) is collecting detailed information on national definitions, statistical recording and crime subcategories. Results will be available mid-2008
For your convenience, here are the two definitions:
"Murder": Any act performed with the purpose of taking human life, in whatever circumstance. This definition excludes abortion but includes infanticide.
Interpol further instructs: Attempted offences are to be included in the number of offences known to the police; in addition, the number of attempted offences should be expressed in the column labelled "Attempts (%)" as a percentage of the total number of offences.
“Intentional homicide” may be understood to mean death deliberately
inflicted on a person by another person, including infanticide
Two separate figures should be provided for "completed" and "attempted"