I'm afraid you're wrong! The chart is exactly right.
The term applies to each school term, not to when your child is born!
The targets, therefore, are for each term of school.
Look at yr 6, autumn term:
The focus group children (orange) ore 3c, 3b and 3a. This is below age expectations. 4c and 4b (white) are securely on target, while 4a and above (blue) are working above expected levels.
Now look at spring term ... the grid has changed slightly, as the children are expected to have made some progress.
By the summer, they are hoping that their target group (orange) have made enough progress to be at 4c, more children will have moved into 4b and 4a (white) and anyone in 5 is working above expected levels so is therefore termed 'more able'.
This is nothing to do with birth dates. It is hard data, what the school is judged on. It is all about getting those children to 4c and above. It is a tool for teachers to track progress and pick up those who are falling behind or staying static. It is part of their target setting process. It's unusual to share with parents, precisely because it is tricky to understand, as you've shown.