This often happens with high performing students.
At the start of the year they are being compared with students who have had no teaching at all or very little. If they can read a bit and know all their letters, can recognise numbers etc., they will score very high compared with the rest of the group.
At the end of the year, all students will have received the required teaching hours. Some will race ahead once they have had some teaching. This means that, although they do better at the end of the year, so do lots of other students, so their standardised scores (which show where they are in the group as a whole) tend to be lower because lots of other children are achieving those scores too.