In my experience, the reading ability of a kid doesn't always follow this nice progression of predictably, gradually getting more advanced. Some kids will plateau for awhile, only to jump back to grade level expectations (or above) out of nowhere.

DS did that when he was 6. He was not an early reader (at all) but suddenly around 6.5 started reading chapter books and was barreling through Harry Potter several months later (with full comprehension). I'm pretty sure that can happen in the older years, too.