Indeed, it seems every school has different policies regarding when a child is moved up; and also, has different numbers of books per level.
At DS' school, they were moved up when they were reading a level confidently in reception, also tied in with their phonics knowledge, to an extent. But in Y1 they were made to read every book in a level before moving. At DS' school they have 45-55 books in each level (up to gold/level 9 - I think they have fewer KS1 books beyond that). So although they got 4-5 books per week, it would take them a whole term to get through a level (what with books starting in week 2 or 3, no books in last week of term, odd days with no books changed, etc) - that is, if they read a book per day. No matter how well they were reading those books.
I'd talk to school without any preconceptions of how things are done elsewhere. I'd be hopeful for a positive outcome to that talk, but yet I'd be prepared to be fobbed off and told patently untrue reasons for why they do things the way they do (e.g. I was told they need to read each and every book in a level because if they don't, there will be gaps in their learning).
And once you know how school handles things, you can suggest changes of course, but my experience is that schools don't like parents suggesting changes in what they see as their expertise. Probably you'll just need to live with it and work around it - so if you feel your child needs different books than what she is being given by school, then provide those books for her (library, reading chest).
But as long as she is happy and making progress, there is absolutely no point in comparing her book levels to other children's book levels, and, if it were possible, even less point in comparing book levels between schools.