Never really saw why deviation wouldn't be side to side to be honest, otherwise, if your aim can be perfect left to right, why not over/undershoot? A couple of degrees of elevation off will deviate a shot, but so will a couple of degrees off left to right. So what's the difference?
Think about it, if targetting is so accurate that your first shot will be spot on laterally, then I don't think you are going to over or under shoot either (our targetting systems are 100% efficient left and right, but can't do basic trig). PLus, you are firing at moving targets for the most part, so I think the random scatter is a bit more realistic, after all, if you fire at a moving bug, it's got to be guesswork as you can't be sure he's going to be in the spot the shells are going to come down in, maybe different rules for artillery versus a static target? Or even a roll for mobile models to simply get out of the way of incoming artillery. (Going by the film and CGI, I can't see a unit of MI standing still while a plasma shot arcs overhead and lands on them). Maybe factor that into reaction? If a model sees incoming artillery it can make a beat feat reaction to get out of the way if possible, then if it's really unlucky it runs into the scattered shot. (also means the unit has used it's reaction).