In the article,The Effectiveness of Child Restraint Systems for Children Aged 3 Years or Younger During Motor Vehicle Collisions: 1996 to 2005,The authors state that their results agree with the other studies except that by Elliott et al. on the relative safety of child seats vs. seat belts in the 2-3 year age group. Based on the description of the Elliott et al study in the introduction, what are some reasons that difference could have occurred?