Acteal massacre weren't registered as homicides in the mortality database and about the large number of deaths of unknown intent in Ciudad Juárez. There's actually another large error in the database that occurred in January and February of 2007. Lots of accidents of unspecified injury mechanism were registered in the Federal District at the same time there was a never before seen drop in the number of homicides.
The drop didn't just involve homicides but also all kind of accidental deaths, though that is not an error but a drop in the quality of vital statistics data (I only show accidental deaths by cause of transportation in the chart for clarity purposes)
It looks as if the error didn't just happen in the Federal District but all over Mexico (though obviously no the state of Mexico). You see the same pattern of a big drop in homicides at the same time accidents by unspecified cause rise. Perhaps someone hired his dump brother in law to administer the database and he screwed things up?
In Nuevo Laredo during the first months of 2007 there were more drug related homicides that total homicides according to the INEGI.