If you have an activity that uses ratings with aggregation set to "count" each student's grade will be set to the number of ratings they have received. For example their grade will be 2 if they have received 2 ratings. If the teacher then the aggregation method, for example to "average", the student's grade won't be updated until they receive another rating (or something else prompts the activity to update student grades).
This means that a student with 2 ratings submitted (grade == 2 while aggregation == count) will have their grade temporarily misreported as being 2 even if the average of those two ratings is not 2.