It makes sense that an area without gun culture produces cops with less skill in shooting. Less prior experience and less instruction from family/friends.
Also less interest in firearms (and thus less practice) outside of work. There are plenty of LEO's I personally know on both sides of that line..all the ones who grew up with firearms are phenomenal shots (and shoot for fun outside of work), those that didn't, don't. I was actually surprised (first knowing more of the former) when I when shooting with some LEO friends and was able to significantly out shoot them...when I asked why, they said they don't get a lot of practice, and just do so to qualify. (note, the former were suburban LEO's mainly, the latter was a DC LEO)