Curious that you didn't bother to supply the link to your stats. Even more curious that you don't see suicide and police killings as forms of gun violence. 

However you miss part of the issue when you ignore the fact that even though those four cities have "strict" gun control, many jurisdictions in the US don't and the free flow of firearms is easily accomplished. In Nevada for example, all you need is a Nevada state drivers license. 

The primary issue in this case is the ease and frequency of "mass shootings" which are defined as four to more victims. There have been 273 such incidents in the US this year. That's almost one a day.

https://www.theguardian.com/us-news/ng-interactive/2017/oct/02/america-mass-shootings-gun-violence