There can be almost a year in age gap for some children starting reception, some have just turned 4 and some are almost 5. So the age bands help us to determine whether this child is achieving below expectations, as expected, or above expectations. It is just a guide. So for a child that is 52 months, I would expect them to be achieving mostly in the 40-60 age bracket, and would be concerned if they were not hitting any of these development points without an identified reason (maybe they speak English as an additional language for example)
So when you say average, yes I guess so. But there is a little more to it than that, because the information needs to be considered in context of the individual child & setting (their chronological age, any special needs or consideration that impact on learning & development, were they full-time, did they have good attendance etc...) I would think of the assessments as guidance for of how the teacher needs to work with that child, rather than an explicit assessment of the child. Because the information is not complete without context.
To expand on this further- when we look at the curriculum area CLL (Communication, language and literacy) You might have a child who can talk in the most complex and complete sentences and picks up vocabulary & understanding of language very quickly. However they are not yet interested in writing or reading. This gives very mixed observations of skill for this one area, for making an assessment. The assessment needs context to be accurate rather than just saying this child is "achieving below expectation for CLL".
From September the curriculum will divide Communication and language from literacy, so this will become less of an issue. Hopefully it highlights that the assessments are only one piece of a very complex puzzle (your child's development & how it relates to the curriculum!) and you should exercise caution when interpreting a single piece of data.
In essence don't worry so much about the points. Listen to the feedback that the teacher gives you about your child. If they don't give much ask questions. If there is an area that you are concerned about- post it here, I can help you form some questions to get at what you need to know to support your child further :)