You cant compare what levels of books other kids are on when they are from different schools.
All schools use the schemes for their own benefits, some will ensure that kids can "read" properly and fully understand the books, other schools will just upgrade the colours because a child has read the book, perhaps once, but isnt really able to understand it.
Some schools change books every day, others might not change the book for a couple of weeks.
Some use the books religiously as a true guide, others use the books as a means to just get kids to read.
There is no point getting anxious, worried, or upset by comparing, because you are not comparing apples with apples between different schools, and sometimes even classes within a school where teachers have different views of the use of the books.
To get general age guides, use the book websites and look at their reference tables.
And, speaking from experience of wanting to know just like you do now, it really doesnt matter down the line. The kids dont get better or worse thought of because of their reading or maths ability. Its more important that children can understand and discuss what they are reading, rather than racing through the colours but not have a clue about what they have read.
Just enjoy reading with your DC.