Maybe something as simple as she moved just as he was pulling the trigger..
At least two dead victims were moving (IIRC, the first two), the guy in the food shopping center parking lot and the guy mowing the lawn at another parking lot.
Here is where I think skill and training could be a discriminant. A sniper is presumeably trained to snipe (or whatever the verb is) well. With a target rich environment that a US suburb is, he or she can afford to find or wait for a stationary target.
A real pro would (I imagine just from light reading) take pride in the bullet to kill ratio, and even the manner of the kill. For example, head shots versus center of mass shots.
Piggybacking here, I have not yet seen any ballistics info released on details such as barrel twist. That might go a long way towards narrowing down the type of .223 weapon involved (and thus aid identification). I am wondering specifically if the police already know but have artificially enlarged the profile in case the sniper has multiple weapons while using only one, or in case a number of people are involved.