I don't know the case, just the article and it seems the cps went with careless/negligent driving, so the judge is obligated to follow sentencing guidelines.
From my experience, guiding my (guilty) cousin through the UK court system from afar, the problem if a judge whacks on a maximum sentence, is that it sets it up for an appeal, that often gets the defendent released if successful. A moderate sentence is far more likely to not be appealed or have the appeal rejected.
The girl's age and gender, and especially if a first offence, would make it unlikely to serve a custodial sentence anyway, so a relatively low one makes it more likely that she'll serve it. If she was hit with five years, for example, the mitigating factors would have her out on appeal.
There's no excuse for her behaviour after, if true. Maybe she is extremely immature and unable to process what happened. The loss felt by the parents of the victims must be agonising and you would think that the defendent will have it on her conscience for the rest of her life.
