v-for index not starting from 0 but less than class_student.id with 1

<tr v-for="(class_student ,index) in class_students" :key='class_student.id'>
                       <td>{{index}}</td>
                       <td>{{class_student.id}}</td>
                       <td>{{class_student}}</td>
                   </tr> 

my question is I need the index to start from 0 as normal and why this happened