I don't know, as others have said there doesn't seem to be an absolute standard of what constitutes an appropriate book band for a child, so there's not much point in taking them too seriously.
There also seems to be a lot of variation in how often children are properly assessed for reading ability, even within a class.
My DD was in Reception last year, and I think it's fairly likely that I know much more about her reading ability and progress than her teacher did. Her reading progress book was only written in twice during the whole year, once by the teacher (during the first fortnight of the first term) and once by the TA at the end of the first term. (I wrote in it several times each week.)
As far as I can make out, apart from these two occasions, she wasn't heard reading by the teacher, the TA or the parents who came in to help with reading, as by their own admission they would focus on the children who weren't getting much help at home.
Another complicating factor is confidence in 'performing' in front of teachers - I know that if DD was to read to a teacher at school, she wouldn't be confident enough to read with the style and expression that she does at home. So it would be very difficult for a teacher to properly assess whether she is actually capable or not. As it happens she is totally capable and a very good, expressive reader, but I would be very surprised if a teacher was able to elicit this in a school environment.
So I guess what I'm trying to say is don't set too much store by which levels your child is 'put' on, as long as you are confident that your child is progressing well and enjoying reading.