I worked in several UK schools as a teacher where it was generally the consensus that children are moved up very conservatively in order to show 'progression' and to hold back until the end of the year to give the impression of more of a jump from one term to the next. It's not right for starters and this isnt even the case here. Its pedagogically unsound to put a cap on a child's learning in this way.
However, the amount of times that parents have complained that their child's book is too easy for them, when that is not the case, is very frequent.
Some children are excellent at decoding even more advanced level books and could probably read the ingredients in the washing powder with ease, but cannot answer more than simple recall questions about what they have read. They can't make inferential predictions or talk about how a character feels. They can just read all of the words. That's one reason why a child may not be moving up as fast as parents would like.
Another reason is that often, children are introduced to a book in school and a teacher supports them to read it. By the time they go home and read it to parents, they have already had one go and remember the story, or parts of it. So even if they were struggling with the first read, they might be a lot more confident the second time around but they are not solid in that book.
Some parents' expectations of what is an example of really secure reading might be different from what a teacher has judged.
Deciding on a reading level is a really fine balance between phonetic knowledge, comprehension, knowledge of words that arent decodable, fluency, expression and confidence. One child might do well by moving up a band as a confidence boost (if they are pretty much on the threshold) while another child might feel more secure to wait and gain a bit more mastery. Myself and colleagues put a lot of thought into getting it just right for every child and we move the children to a band that is right for them, not stopping at the expected level. Read loads of books at home because that's the most important thing and the ultimate aim is for an independent love of reading anything. The banded books are just a tool to get there.